From 1967-82, 9 children with testicular relapse (TR) of acute lymphoblastic leukaemia (ALL) were diagnosed out of 99 boys treated, an incidence of 9.1%. The median time from the onset of ALL until diagnosis was 28 months (range 3-41 months). All were asymptomatic; six were detected on routine examination while three were diagnosed only on biopsy. Routine biopsy prior to stopping chemotherapy is useful in detecting occult TR. Biopsies should be done on both the testes regardless of the clinical findings. The age, leucocyte count and hepatosplenomegaly at diagnosis of ALL were not found to be significant factors in influencing relapse. Eight children were in bone marrow remission at the time of TR, but three had preceding or concurrent meningeal leukaemia while in the other five the testis was the first and only site of relapse. Radiotherapy was effective in local disease control but failed to prevent bone marrow relapse in all except two patients despite continuation of chemotherapy. The median time from onset of TR until bone marrow relapse was 7 months (range 3-13 months) and the median time until death, was 11 months (range 6-18 months). The frequency of testicular relapse may be related to the intensity of either the initial induction therapy or the consolidation chemotherapy. Further studies are required to determine whether the incidence of testicular relapse will decline with more intensive early treatment.