An open evaluation was undertaken to determine the efficacy, safety, and palatability of a spearmint-flavored nystatin lozenge in the treatment of oral candidiasis in cancer patients. Patients were instructed to dissolve two lozenges containing 100,000 IU of nystatin in the mouth, one after the other, four times a day for a seven-day course. Forty-five patients with a clinical diagnosis of thrush were entered in the study. Thirty-six had both culture proof of candidiasis and adequate follow-up information to assess response. Overall, 28 of the patients (78%) benefited from treatment: 22 patients (61%) had complete resolution of signs and symptoms, but only ten (28%) were culture negative at the end of the seven-day course. Twenty-three of the 26 patients (88.5%) who completed seven days of nystatin had clinical improvement and only three (11.5%) failed to improve. These latter patients had a lower mean granulocyte count pretreatment compared with responding patients (1.2 x 109/L versus 5.3 x 109/L), although this was not statistically significant (P = 0.06, Wilcoxon 2-sample test). It was noted that the mean posttreatment granulocyte counts were significantly lower at the completion of the trial in those patients who had a seven-day trial of nystatin but who failed to respond compared with those who responded (1.0 x 109/L vs 5.3 x 109/L; P = 0.02). Ten patients completed less than the minimum course of seven days. Nonetheless, five had a successful outcome, even though they had only an average of 3.2 days of treatment. Those failing nystatin had a mean treatment duration of just three days. Side effects consisting primarily of mild gastrointestinal upset occurred in 12 of 42 patients (29%), and 33% disliked the spearmint flavor of the lozenge. We conclude that nystatin is effective in the treatment of oral candidiasis in cancer patients who experience short duration myelosuppression during chemotherapy. For those patients in whom symptomatic oral candidiasis occurs during a period of prolonged myelosuppression, a systemically administered antifungal agent may be a more effective therapeutic approach.