Fungal infections are an important cause of morbidity and mortality in immunocompromised patients. Treatment with amphotericin B is the main therapeutic approach. However, this treatment is limited by the substantial toxicity. We present the data of the first randomized prospective comparative trial in adults (134 patients with fever of unknown origin) with conventional amphotericin B and a liposomal formulation of amphotericin B (AmBisome, published in 1997 by Prentice et al. (Br. J. Haematol. 98, 711-718) and the data of adults with documented fungal infections (59 patients), treated in this trial. Patients received either conventional amphotericin B 1 mg kg-1 per day, liposomal amphotericin B 1 mg kg-1 per day or liposomal amphotericin B 3 mg kg-1 per day. Patients were entered if they had fever of unknown origin (FUO), defined as temperature of 38 degrees C or more, not responding to 96 h of systemic broad-spectrum antibiotic treatment, and neutropenia (< 0.5 x 10(9) l-1). Efficacy of treatment was assessed, with success defined as resolution of fever for three consecutive days (< 38 degrees C) in the group of patients with FUO and the freedom of clinical signs and/or the elimination of fungus in the group of patients with documented fungal infections. The safety of treatment and renal and hepatic toxicity of liposomal and conventional amphotericin B were compared. No statistically significant difference was found in the treatment efficacy in the three study arms. However, there is a tendency of better treatment results in the two groups of patients, who received liposomal amphotericin B. Thirty-five per cent of patients with documented fungal infections and 46% of patients with FUO responded to amphotericin B. In the patients group, that received 1 mg kg-1 liposomal amphotericin B it was 63 and 49%, in the group of patients that received 3 mg kg-1 liposomal amphotericin B it was 47 and 64%. Evidence of toxicity due to amphotericin B was seen in 50 patients (83%), toxicity due to liposomal amphotericin B, 1 mg kg-1, was seen in 35 patients (50%), and due to liposomal amphotericin B 3 mg kg-1 in 34 patients (54%). This was a statistically significant difference (P = 0.001). It was concluded that liposomal amphotericin B was safer than conventional amphotericin B, but both formulations are equivalent in treatment efficacy. The prophylactic use of amphotericin B in these immunocompromised patients is discussed.