The incidence of second malignant neoplasms (SMN's) was investigated in a group of 529 patients with Hodgkin's Disease (HD) treated at St Bartholomew's Hospital (SBH). SMN's were seen in 27 of these patients giving an incidence rate three and a half times that expected in an age and sex matched normal population (p = much less than 0.001). The incidence rate was higher in those receiving multiple chemotherapy and radiotherapy for relapsed HD compared with those receiving primary radiotherapy, chemotherapy or chemotherapy with adjuvant radiotherapy (p = 0.02). However, the increased incidence rate in those patients treated with chemotherapy on relapse, may reflect in part a delayed effect of their primary therapy, since the incidence rate in the primary treatment group only becomes significantly raised after six years. When allowance was made for this delay the difference between the two groups was no longer significant. The incidence rates for Non-Hodgkin's Lymphoma (NHL) and myelogenous leukaemia were 32 and 57 times those expected, compared with only two and a half times the expected rate for non-haematological SMN's (p = much less than 0.001). The four acute myeloid leukaemias (AML) all occurred within five years of treatment compared to wide-ranging intervals between treatment and occurrence of SMN in the other groups. The increased incidence of NHL may be an alternative expression of lymphoid abnormality rather than a treatment-related occurrence. Multiple SMN's were diagnosed in three patients. This represented a highly significant (p = much less than 0.001) increase over the expected incidence of multiple neoplasia in the general population. Several factors may contribute to the development of SMN's in HD, including an inherent disposition of HD itself. The time-dependent incidence pattern of SMN's with a delay followed by an increased incidence rate, suggests that treatment plays a key role. It is not yet clear whether more intensive, or multiple treatments add to the risk accrued for the initial treatment.