Systemic yeast infections are a major cause of morbidity and mortality in severely immunocompromised patients. The in vitro susceptibility to amphotericin B of 29 yeasts causing fungemia was examined in 26 patients undergoing allogeneic or autologous bone marrow transplantation and/or myelosuppressive chemotherapy. The minimal inhibitory concentrations (MICs) of amphotericin B observed with blood isolates from these patients were significantly higher than those observed with blood, sputum, or skin isolates from non-immunocompromised patients (p less than 0.01). All episodes (10 of 10) of bloodstream infection in immunocompromised patients caused by isolates with MICs greater than 0.8 micrograms/ml were fatal, versus eight of 17 episodes of bloodstream infection caused by yeasts with MICs of 0.8 micrograms/ml or less (p = 0.04). Although 15 of 26 patients received empiric treatment with amphotericin B before laboratory evidence of fungemia developed, the amphotericin B susceptibilities of their isolates were not significantly different from those of patients who had not received empiric amphotericin B treatment. It is concluded that yeast fungemia in severely immunocompromised patients is often caused by organisms resistant to the usual concentrations of amphotericin B obtainable in vivo, and that this finding is clinically significant.