We conducted a randomized, double-blind trial of 1- versus 4-h infusions of amphotericin B to determine whether there was any difference in infusion-related toxicity. A total of 128 maintenance infusions in 12 patients were studied; 62 were randomized to 1-h infusions (group A) and 66 were randomized to 4-h infusions (group B). We found no significant differences between patients in groups A and B in mean temperature, pulse, or systolic or diastolic blood pressure measured during the infusions. At a significant level of 0.05, the power to detect a mean difference in temperature of 2 degrees C, a pulse difference of 20 beats per min, a decrease in diastolic blood pressure of 10 mm Hg, or a decrease in systolic blood pressure of 20 mm Hg was 0.95. Rigors and chills were noted in 15 of 62 (24.1%) infusions in group A patients and 12 of 66 (18.1%) infusions in group B patients (P = 0.40). Meperidine was required because of severe persistent rigors in 6 of 62 (9.6%) infusions in group A patients and 6 of 66 (8.9%) infusions in group B patients (P = 0.91). An increase in temperature was noted in five (8%) of the group A infusions and seven (10.6%) of the group B infusions (P = 0.63). The mean time to onset of rigors, an increase in temperature, and an increase in pulse occurred significantly earlier in group A than in group B patients (P = 0.02 for all comparisons). We conclude that there is no difference in the incidence or severity of the infusion-related toxicity of amphotericin B with a 1-h infusion rate compared with a 4-h infusion rate. However, the onset of infusion-related toxicity occurs significantly earlier with a 1-h infusion.