Flucytosine is an antifungal agent useful in combination with amphotericin B in the treatment of several deeply invasive mycoses. The potentially dose-limiting, hematologic, gastrointestinal, and hepatic toxicities of flucytosine lead to a reluctance to use it in myelosuppressed patients. To investigate the safety and tolerability of flucytosine in this setting, we evaluated its use in 17 patients with cancer or aplastic anemia during a 2 1/2-year period at our institution and reviewed the literature describing mechanisms of action, resistance, in vitro and in vivo antifungal activity, clinical antifungal activity, pharmacokinetics, and toxicity. The combination of amphotericin B plus flucytosine eradicated the mycosis in 12 (71%) of 17 patients, whereas 3 (18%) of 17 died of progressive fungal infection. Serial serum levels of flucytosine measured by a creatinine iminohydrolase assay permitted reliable dosage adjustment. During therapy, only 2 (12%) of 17 patients had elevated mean serum levels of flucytosine (> 100 micrograms/mL) and 3 (18%) other patients had transiently elevated levels. Paired serum samples (n = 45) obtained at steady state during therapy with orally administered flucytosine showed similar peak and trough levels. Adverse effects of flucytosine therapy included one case each of reversible nausea, diarrhea, elevated transaminase levels, and thrombocytopenia. No cases of bone marrow aplasia, enterocolitis, hepatitis, or death due to flucytosine toxicity were encountered. We conclude that flucytosine in combination with amphotericin B is well tolerated in myelosuppressed patients when serum flucytosine levels are serially monitored.