BACKGROUND: Adjuvant chemotherapy has improved the outcome of childhood malignant brain tumors in large randomized trials. With increasing survival rates, treatment toxicity has become a matter of concern. Radiation therapy and cisplatinum are known to be ototoxic. METHODS: We evaluated the incidence and factors predisposing to ototoxicity in children treated with the "8 in 1" chemotherapy protocol in Finland during 1986--1993. Thirty-five of the 82 children survived for at least 1 year after diagnosis. Thirty of these children were old enough to have an audiogram. RESULTS: Seventeen of the 30 children had normal hearing, seven had hearing loss at high frequencies, and six (20%) had severe hearing loss in the speech range. The risk factors for severe hearing loss were young age, a high cumulative dose of cisplatinum, and deteriorating renal function. In the presence of these factors, the risk of severe hearing loss was over 50%. Hearing loss at high frequencies could occur after low cumulative doses of cisplatinum, but severe hearing loss correlated with high cumulative doses. CONCLUSIONS: Cisplatinum-induced hearing loss at high frequencies is common, but hearing loss in the speech range also occurs, particularly in children with predisposing factors, and may progress insidiously and rapidly. Therefore a hearing test before each "8 in 1" course is important.