The authors report on three cases of thymic seminoma treated between 1971 and 1981. These tumours, first described by Friedman in 1981. These tumours, first described by Friedman in 1951, belong to the group of extra-gonadal germinal tumours. They constitute about 2.5% of all thymic masses. The most probable pathogenic theory is abnormal migration of germinal cells from the vitelline sac to the embryonic thymus. Thymic seminomas are usually found in young men and are asymptomatic in 30% of the cases. Macroscopically, they present as solid tumours capable of invading the surrounding structures. Histologically, they resemble gonadal seminomas but are sometimes difficult to identify, which is unfortunate since treatment is dependent upon an accurate histological diagnosis. The authors suggest that the tumour should be biopsied under mediastinal fluoroscopy, so that an accurate histological diagnosis can be made. Treatment consists of surgical excision, which should be restricted and on no account should destroy important structures, completed by mediastinal radiotherapy. The mean survival time is 6.3 years; the 5-year survival rate is 75%.