The histological and clinical findings in five cases of thymoma arising in paediatric patients have been studied. The age range was 11-15 years and no patient was affected by myasthenia gravis. All tumours were macroscopically encapsulated, but two of them displayed evidence of microscopic capsular invasion. Histologically, four cases were of the predominantly cortical type (organoid thymoma) with prominent areas of medullary differentiation and Hassall's bodies; one case was of the cortical type. All patients are alive and disease-free 3 months to 9 years after surgery. These findings suggest that thymoma in the paediatric age group may be characterized by fairly uniform clinicopathological features, with a low rate of association with myasthenia gravis and a favourable prognosis.