Disseminated Penicillium marneffei infections are common AIDS-defining opportunistic infections among persons with human immunodeficiency virus (HIV) infection in northern Thailand. Penicilliosis due to P. marneffei is the third most frequent AIDS-defining infection in this population, after tuberculosis and cryptococcosis. Very little is known about the epidemiology and natural reservoir of P. marneffei. The seasonal distribution of two common AIDS-defining fungal infections was compared among patients diagnosed between 1991 and 1994 at Chiang Mai University Hospital. There were 550 cases (492 male, 58 female patients) of P. marneffei and 793 cases (685 male, 108 female patients) of Cryptococcus neoformans infection diagnosed. In each year, P. marneffei but not C. neoformans infections were more frequent in the rainy than the dry season. Seasonal variation of P. marneffei infections in AIDS patients in northern Thailand may provide valuable information in determining the important reservoirs and exposures to this organism that lead to disseminated disease in these patients.