The aim of this study was to assess the value of serum inhibin B in detecting male gonadal dysfunction in childhood cancer survivors. 27 male postpubertal (Tanner's pubertal stage G5 or P6) and 12 pubertal (> or = G2) patients were drawn from the endocrine follow-up protocol of childhood cancer patients at the Paediatric Clinic of Turku University Hospital, Turku, Finland. The average time (mean +/- S.D.) between the diagnosis and this study was 11.7 +/- 4.5 years in the postpubertal and 7.0 +/- 3.9 years in the pubertal group. Serum samples for the determination of follicle-stimulating hormone (FSH), luteinising hormone (LH), oestradiol, testosterone, and inhibin A and B dimers were collected. The demographic factors, pubertal stage and testicular size of the patient were measured at the same routine outpatient visit. Serum inhibin concentrations were correlated to testicular volume and gonadotrophin concentrations. Strong correlations were observed between testicular size (r = 0.80, P < 0.001) or FSH (r = -0.58, P = 0.002) and inhibin B concentration in the postpubertal group. Inhibin A was not detectable (< 2 pg/ml). Testicular volume measurement was accurately documented in 21 postpubertal subjects. Patients with small testicles (< 10 ml) had inhibin B concentrations under 42 pg/ml and those whose testicular size was over 13 ml had inhibin B concentrations exceeding 100 pg/ml. In all 12 pubertal survivors, serum inhibin B levels were > or = 94 pg/ml, except in one case of testicular cancer where inhibin B was 8.1 pg/ml and the FSH concentration was elevated. Inhibin B seems to be an indicator of male gonadal function in postpubertal childhood cancer survivors and could be used in the estimation of gonadal function of male survivors earlier than testicular volume or semen analyses would be routinely possible. However, the correct cut-off level of serum inhibin B, as well as the details of inhibin B physiology during puberty, remain to be determined before semen analysis can be replaced by the measurement of inhibin B.