UPDATED: February 16, 2012, at 9:55 p.m.

Allan M. Brandt announced Wednesday that he will be stepping down from his role as dean of the Graduate School of Arts and Sciences due to health considerations stemming from a recently diagnosed illness.

“I am deeply grateful for the opportunity I have had to know and collaborate with all of you,” Brandt wrote in an email to his GSAS colleagues. “The core mission you have worked hard to support will continue unchanged.”

Noting his leadership capacity and positive energy, faculty members said news of Brandt’s decision was unexpected and sobering.

“We were devastated,” said Garth O. McCavana, GSAS dean for student affairs. “We were very sad...that he is stepping down, because he is such a great leader.”

Faculty of Arts and Sciences Dean Michael D. Smith highlighted Brandt’s accomplishments in pedagogical innovation, diversity at the Ph.D. level, and student support in an email that he sent to FAS faculty members and staff on Thursday.

“In the five admissions cycles he has overseen, the quality of our entering Ph.D. and A.M. [Masters of Arts] classes has risen to unprecedented heights, as have the number of applications to GSAS,” Smith wrote.

In addition to developing the student body of GSAS, Brandt launched Graduate Seminars in General Education—a program through which graduate students help design undergraduate courses.

“He sees undergraduate and graduate education as part of the same seamless fit,” said classics professor Richard F. Thomas, who serves as the classics department’s director of graduate studies.

Colleagues praised Brandt for increasing the accessibility of graduate education at Harvard.

Applications to GSAS—including those from underrepresented minorities—have increased consistently during his tenure.

And in the midst of the financial downturn since 2008, Brandt has continued to fund graduate stipends and student support programs.

“He has created an unbelievably collaborative and communicative environment among the directors of graduate studies,” said Marie D. Dahleh, director of graduate studies for engineering and applied sciences. “We’ve been getting together more frequently...to discuss concerns that go beyond departmental boundaries.”

Brandt also made an effort to interact with the Graduate Student Council—the school’s representative student body.

Council President Cammi N. Valdez said that Brandt’s dedication to graduate students went “above and beyond” the call of duty. “Finding someone who is as committed to graduate students as Dean Brandt is rare,” Valdez said.

Beyond his role as an administrator, Brandt has become known for his warm personality and genuine dedication to the needs of his colleagues and of his students.

“He’s a truly fantastic fellow,” said Eli Tziperman, director of graduate studies for earth and planetary sciences.

In addition to his administrative role, Brandt holds joint appointments at Harvard Medical School and the history of science department.

“[Teaching] is something he’s been able to continue actively during the time he’s been dean,” said James M. Hogle, Dudley House Master and director of graduate studies for biophysics. Hogle added that he appreciated that Brandt was an “ardent supporter” of Dudley House.

Although Brandt does not plan to return as dean, he does intend to resume teaching duties upon finishing medical treatment.

“I will greatly look forward to my return to the faculty and to re-engaging in the critically important work of graduate education,” he wrote in his email.

Richard J. Tarrant, Pope Professor of the Latin Language and Literatures and acting dean of GSAS from 1995 to 1996, will serve as interim dean while Brandt’s replacement is found.

Smith and Brandt approached Tarrant last weekend to request that he serve as interim GSAS dean. Tarrant accepted the appointment after conversations with both administrators on Tuesday.

“My highest priority is to ensure as smooth a transition as possible, so that the work of GSAS can be successfully carried on through the end of the academic year,” Tarrant wrote in an email to the Crimson.

“He knows the graduate school well and the administration well,” said Margot N. Gill, GSAS administrative dean.

The search for Brandt’s permanent replacement is in its initial stages, according to FAS spokesperson Jeff Neal. Smith has invited members of the FAS community to send him their thoughts on potential candidates.

—Staff writer Radhika Jain can be reached at radhikajain@college.harvard.edu.

—Staff writer Kevin J. Wu can be reached at kwu@college.harvard.edu.