Diphenylhydantoin-induced hypogammaglobulinemia in a patient infected with human immunodeficiency virus. A case is reported of reversible panhypogammaglobulinemia in a human immunodeficiency virus (HIV)-infected patient. Onset and resolution were temporally correlated with initiation and termination, respectively, of diphenylhydantoin therapy for a possible seizure. A rapid alteration in peripheral T-cell subpopulations was also noted in association with diphenylhydantoin administration. This case is compared with previous reports of diphenylhydantoin-associated hypogammaglobulinemia in non-HIV-infected patients. In addition, the case is discussed with regard to possible deleterious effects associated with the use of diphenylhydantoin as therapy for HIV-associated seizures or as an antiretroviral agent in HIV disease. 