The morphological appearance, incidence and prognostic significance of human chorionic gonadotrophin (HCG)-containing cells in seminomas were examined in a retrospective series of 228 orchidectomy specimens, obtained between 1958 and 1972. Sections from each tumour were stained with haematoxylin and eosin (H & E) and immunocytochemically for HCG. In 33 (14.5%) of the tumours HCG-containing cells were observed, but in only 12 were these recognised in an initial study of the H & E stained sections. HCG staining was seen predominantly in syncytiotrophoblastic giant cells and rarely in "mulberry" cells and mononuclear seminoma cells. Of the patients whose tumours included HCG-containing cells 23% died of their disease within 2 years of orchidectomy, compared with only 8% of the patients whose tumours lacked this feature. It is concluded that immunocytochemical staining for HCG should form part of the routine histological assessment of seminomas, and that the presence of HCG-containing cells indicates a more aggressive disease.