This study was undertaken to determine the treatment-specific incidence of second malignant tumours (SMT) in childhood Hodgkin's disease. The institutional databases at The Hospital for Sick Children, the Princess Margaret Hospital, and the Toronto-Bayview Regional Cancer Centre were reviewed for the years 195b1993. Three hundred and forty-three consecutive newly diagnosed children were evaluated. The overall 30 year cumulative SMT incidence was 31%. The 20 year SMT incidence was greater for patients who relapsed (n = 129), 27%, compared with patients who remained relapse free (n =214), 13%. For patients with stage 1-38 disease who remained relapse free, the 10 year SMT rate was 7% for patients who were surgically staged and treated with extended field radiation treatment (EF RT) (35 C ) , compared with 3% in clinically staged patients treated with MOPP (six cycles) and EF RT (25-30 G ) . To date there is no significant difference in the oncogenicity of these treatment protocols. However, EF RT alone was less effective in disease control. For stages 7-38, 62% of patients relapsed after EF RT alone compared with 18% after bimodal treatment. Therefore treatment intensification due to relapse was more frequent in the former group. The overall 10 year SMT incidence for patients treated with these protocols was 11% and 3%, respectively. The 20 year SMT incidence following EF RT alone was 24%. We conclude that SMTs were a common late complication in childhood Hodgkin's disease and are a limiting factor in the achievement of cure. The incidence of SMTs was increased in children who required retreatment and was minimal in children who remained in a first complete remission. Therefore the initial treatment strategy in childhood Hodgkin's disease must be to minimize the risk of relapse, in order to avoid the morbidity and mortality associated with both relapse and SMT induction, and to achieve this objective with a primary treatment protocol of low oncogenicity.