Survival and endocrine status in a cohort of boys with acute lymphoblastic leukaemia (ALL) who started treatment between 1972 and 1987 and subsequently developed a testicular relapse were analysed. During this period there was a significant improvement in the overall event free survival for boys, but no significant decrease in the testicular relapse rate. Thirty three boys had an apparently isolated testicular relapse, whereas 21 boys had a combined relapse. The event free survival for boys with an isolated testicular relapse was 59% at six years (95% confidence interval (CI) 42 to 74%). The event free survival for the 16 patients with a combined relapse who received a second course of treatment was 32% (95% CI 17 to 60%). Those patients receiving adequate second line treatment for an isolated testicular relapse whose first remission was longer than or equal to two years had an event free survival of 82% (95% CI 63 to 93%) at six years. No boy relapsing within two years from diagnosis has survived. Endocrine late effects are significant, with 82% of the boys requiring hormonal treatment at some stage for induction of puberty or continuing pubertal maturation, or both. It is concluded that, despite the increasing intensity of initial treatment for ALL, isolated testicular relapse is treatable by conventional means in most patients. Careful endocrine follow up of these patients is essential as most will require hormone replacement treatment.