Summary Testicular function was studied in 40 males treated in childhood for Hodgkin's disease at St Bartholomew's Hospital, and the Hospital for Sick Children, London, between 1971-1985. All patients were 16 years or over at evaluation, and off treatment more than 6 years. Basal FSH, LH and testosterone levels were measured. Testicular size was measured using a Prader orchidometer, and all patients were offered a seminal analysis. Twenty-eight patients were treated with chemotherapy, usually ChlVPP. Twenty-one also had radiotherapy, five below the diaphragm. Twelve patients were treated with radiotherapy alone (five below the diaphragm). Twenty-six of 28 patients treated with chemotherapy and three of five patients treated with radiotherapy alone below the diaphragm have elevated basal FSH levels, and 18 of these also have elevated basal LH levels. Median testicular volume is 11 ml (range 5-25 ml). Eleven With a current 5 year event free survival of 90%, there is increasing interest in the late effects of treatment in patients with Hodgkin's disease. Decreased sitting height due to extended field irradiation including the spine (Wilimas et al., 1980) and abnormalities of thyroid function after neck radiotherapy have been well documented (Shalet et al., 1977). Infertility appears to be almost inevitable in adult males with Hodgkin's disease treated by six or more courses of MOPP or MVPP (mustine, vincristine/vinblastine, procarbazine, prednisolone) (Whitehead et al., 1982a). Chemotherapy induced testicular damage in patients treated for Hodgkin's disease in childhood was first reported by Sherins et al., in 1978. Whitehead et al. (1982b reported 15 males treated with MOPP for Hodgkin's disease in childhood and concluded that severe testicular damage is common, with azoospermia, but normal pubertal development. More recently Bramswig et al. (1990) evaluated testicular function in 75 boys treated for Hodgkin's disease with involved or extended field irradiation and chemotherapy with OPPA (vincristine, prednisone, procarbazine, doxorubicin) or COPP (cyclophosphamide, vincristine, prednisone, procarbazine). Testicular dysfunction was observed in boys treated before as well as during puberty. Abnormal basal FSH/LH levels were found more frequently in patients who had received higher cumulative doses of chemotherapy.We have studied testicular function in 40 males treated in childhood for Hodgkin's disease with chemotherapy (usually chlorambucil, vincristine, procarbazine, prednisolone -ChlVPP) (Robinson et al., 1984) and/or radiotherapy to assess the effect of this treatment on subsequent fertility. range 6-18 years). Basal FSH, LH and testosterone levels were measured. FSH and LH levels were measured by standard immunoradiometric assays with intra-inter-assay coefficient of variation (CV) of < 5% for both assays. Testosterone was measured by radioimmunoassay with a CV of <5%. Testicular size was measured using a Prader orchidometer, and all patients were given the opportunity to have a seminal analysis performed...