Between June 1977 and December 1978, occult testicular leukemia (OTL) was discovered at three years of continual complete remission (CCR) from the time of diagnosis of acute lymphoblastic leukemia (ALL) in 5 of 59 (8.5%) of males undergoing bilateral wedge testicular biopsy at 1 of 15 participating Southwest Oncology Group (SWOG) institutions. Forty-six of the 54 males with normal biopsies (78% of the total group of 59) have remained free of recurrent ALL at a median of 18 months (range 13 to 23 months) since the biopsy procedure, whereas eight have relapsed for the first time--five bone marrow (BM), one sclera, one simultaneous BM and testes, and one testes--at a median of 12.5 months (range 4 to 22 months) after the normal testicular biopsy. With aggressive therapy after biopsy in the five boys with OTL, one has died 19 months after biopsy (after two BM relapses), one is alive 21 months after biopsy (after two BM relapses), and three are alive and free of recurrent ALL 13, 16, and 19 months, respectively, since the diagnosis of OTL.