A profound and long-lasting reduction in circulating CD4+ T lymphocytes (< 80/microliters) was found in a 37-year-old man (without known risk factors for HIV infection) presenting with recurrent oral candidiasis who subsequently developed cryptococcal meningitis. Infection with HIV was ruled out by serological and virological studies. In vitro and in vivo cell-mediated immunity was severely impaired. Abnormal phenotypic patterns of both CD4+ and CD8+ cells were consistently observed. A systematic family survey revealed in some of his asymptomatic relatives several immunological abnormalities which may point to a genetically based primary immunodeficiency disorder.