Twenty-two of 3033 patients with Hodgkin's disease (HD) randomised into the clinical trials of the BNLI have developed non-Hodgkin's lymphomas (NHLs) at periods up to 16 years after presentation (1 simultaneous and 1 composite), giving an incidence of 0.7%. The frequency of NHL varied from 3.8% in lymphocyte-predominant HD to 0.3% in nodular sclerosing HD. In this series, 16 patients developed high-grade NHL (12 B cell; 4 peripheral T cell) and 6 developed low-grade NHL (all B cell). The histological subtype of NHL did not appear to be related to initial histological subtype of HD or the treatment received. In histological subtypes other than lymphocyte predominant, there was commonly evidence of immunosuppression in the form of low presentation lymphocyte counts, advanced stage and systemic (B) symptoms. The results suggest that these patients have a propensity for lymphoproliferative disorders, possibly associated with some immune deficiency and the subsequent development of NHL is not treatment related. The findings also emphasise how important it is to biopsy recurrent disease.