Potassium hydroxide-concentrated smears, prepared from sedimented remains of clinical specimens, were used to distinguish between mold infection and exogenous contamination in fungal culture-positive specimens. This method was applied in the study of 3,857 clinical specimens from 230 bone marrow transplant recipients who were followed up prospectively for infectious complications. Concentrated smears of only 86 (from 21 infected patients) of 149 fungal culture-positive specimens were positive for hyphae; 82 of the strains were Aspergillus species. Concentrated smears of the remaining 63 fungal culture-positive specimens were negative; the strains identified by culture were considered as exogenous contaminants (87% of which were Penicillium species). A stage-specific manifestation of mold infection was observed: 67% of mold infections occurred during acute graft-vs.-host disease (GVHD) a median of 47 days after transplantation, whereas 9% of mold infections occurred as rapidly fatal invasive disease before engraftment. Overall, of the 21 patients with mold infection, 17 (81%) had invasive mold disease, and four (19%) had mold colonization of airways secondary to chronic GVHD after day 100. The significant risk factors for mold infection were total-body irradiation and grade 2-4 acute GVHD. Because of our high mortality rate (82%), the consideration of antimold prophylaxis for such patients may be warranted.