The efficacy of itraconazole, a new triazole antifungal agent, was studied in vitro and assessed in patients. The MICs of itraconazole for 16 strains of Aspergillus fumigatus were in the same range as those of amphotericin B: less than 0.09-0.36 microgram/ml vs. less than 0.09-0.78 microgram/ml, respectively. Eight adult patients with systemic fungal infections were treated orally with 100-200 mg of itraconazole two times a day. A patient with relapsing histoplasmosis (Histoplasma capsulatum var. duboisii) was cleared of the infection; a patient with arthritis of the knee due to Phialophora parasitica did not respond to treatment; four patients with semiinvasive pulmonary aspergillosis improved dramatically and were considered clinically cured; and two patients with aspergilloma improved. The duration of follow-up was one to nine months. Levels of itraconazole in body fluids were measured by a bioassay. Levels of drug in knee fluid were about 30% of the simultaneous levels in plasma. A progressive decrease in the level of itraconazole in plasma occurred in two patients, and a progressive increase in the levels occurred in five patients.