We quantitatively studied the orointestinal yeast colonization of 15 consecutive paediatric patients who underwent 16 bone marrow transplantations (BMT). Cultures were performed initially, longitudinally weekly during the period of aplasia (in-patient treatment) and, if possible, also during out-patient follow-up. With one exception, all patients received fluconazole as antifungal prophylaxis. Patients remained free of yeasts during the complete observation period only in six out of 16 cases (38%). Non-albicans species of Candida were isolated in six out of 16 cases (38%), mainly C. glabrata (five out of 16; 31%). All of these patients had undergone allogeneic BMT. In one case, there was indirect evidence of systemic invasion by C. glabrata. Even combined prophylaxis with fluconazole and and amphotericin B suspension could not reliably prevent yeast colonization but this combination at present appears to be the optimal regime. Regular concomitant Candida serology (determination of specific antibodies by three methods) proved to be a valuable additional surveillance method.