Isolated testicular relapse complicating first hematologic remission was identified in 31 of 521 boys with acute lymphocytic leukemia (ALL). Three categories of involvement were apparent and could be related to presenting clinical features, duration of initial complete remission, and length of hematologic remission. Among 12 patients with early testicular relapse, most had unfavorable prognostic features when ALL was first diagnosed. All but two of these children experienced marrow recurrence within seven months of testicular relapse. In contrast, the 12 patients who developed testicular disease late in their clinical course have responded much better to further therapy; ten remain in bone-marrow remission for a median of four years beyond testicular relapse. Similarly, five of the seven patients with subclinical testicular leukemia, found at elective biopsy, continue in marrow remission for prolonged periods. Early testicular recurrence is a sign of drug-resistant disease; late recurrence after elective cessation of therapy may represent residual, incompletely treated but still responsive leukemia.