BACKGROUND: Despite several investigations, second malignancy risks (SMR) following radiotherapy alone (RT), chemotherapy alone (CT) and combined chemoradiotherapy (CRT) for Hodgkin's lymphoma (HL) remain controversial. PATIENTS AND METHODS: We sought individual patient data from randomised trials comparing RT versus CRT, CT versus CRT, RT versus CT or involved-field (IF) versus extended-field (EF) RT for untreated HL. Overall SMR (including effects of salvage treatment) were compared using Peto's method. RESULTS: Data for between 53% and 69% of patients were obtained for the four comparisons. (i) RT versus CRT (15 trials, 3343 patients): SMR were lower with CRT than with RT as initial treatment (odds ratio (OR) = 0.78, 95% confidence interval (CI) = 0.62-0.98 and P = 0.03). (ii) CT versus CRT (16 trials, 2861 patients): SMR were marginally higher with CRT than with CT as initial treatment (OR = 1.38, CI 1.00-1.89 and P = 0.05). (iii) IF-RT versus EF-RT (19 trials, 3221 patients): no significant difference in SMR (P = 0.28) although more breast cancers occurred with EF-RT (P = 0.04 and OR = 3.25). CONCLUSIONS: Administration of CT in addition to RT as initial therapy for HL decreases overall SMR by reducing relapse and need for salvage therapy. Administration of RT additional to CT marginally increases overall SMR in advanced stages. Breast cancer risk (but not SMR in general) was substantially higher after EF-RT. Caution is needed in applying these findings to current therapies.