The activity of itraconazole in vitro was evaluated for 2,094 strains of 132 fungal species, one achloric alga, nine actinomycetes, and six bacterial species. Itraconazole was active against dermatophytes (271 strains), Candida species (1,303), Cryptococcus neoformans (27), Torulopsis species (170), Pityrosporum species (40), Aspergillus species (87), Sporothrix schenckii (12), dimorphic fungi, Dematiaceae, and various other organisms. This activity depended largely on the test conditions and the medium used. Ittraconazole was as active as ketoconazole in the treatment of dermatophytoses and of both superficial and deep candidosis at oral doses about eight and four times lower, respectively, than the doses of ketoconazole required. Disseminated dermatophytosis was cured more rapidly by itraconazole than by ketoconazole. Parenteral and oral itraconazole were of equal efficacy for the treatment of systemic candidosis. Itraconazole used topically was more active than reference compounds against microsporosis, trichophytosis, and superficial candidosis. Given orally, itraconazole was effective therapy for aspergillosis and meningo-cerebral cryptococcosis in mice and for generalized cryptococcosis, histoplasmosis, and sporotrichosis in guinea pigs. No drug-related adverse effects were observed.