The bone marrow of 22 untreated children with acue lymphocytic leukemia were examined cytochemically with periodic acidSchiff (PAS) reaction. PAS positivity was correlated with relapse rate and duration of complete remission. 18 months after achieving remission 10 of 12 patients with strong PAS-positive marrow at the time of diagnosis are still in complete remission in contrast to only 3 of 10 patients with weak PAS-positive marrow. Of the 10 patients with weak PAS positivity one did not achieve remission, five developed complete (central nervous system, CNS, and hematologic) relapse and one relapsed in the CNS. Of the 12 patients with strong PAS-positive initial marrow, 10 remain in complete continuous remission, one had hematologic relapse and one developed meningeal leukemia. From these data we conclude that PAS positivity is directly related to the duration of initial remission in children with acute lymphocytic leukemia.