AbstractA high frequency of conversion from L1 to L2 morphology in ALL at relapse has been reported in studies made without benefit of the French‐American‐British (FAB) scoring system. We compared the lymphoblast morphology at presentation and first hematologic relapse for 50 consecutive children with acute lymphoblastic leukemia using the FAB scoring system to determine the frequency of morphologic conversion. Forty‐eight patients presented with L1 morphology, 42 of whom (88%) relapsed with L1 blasts, four (8%) with L2 blasts, and two (4%) with blasts intermediate between L1 and L2. Two presented with L2 morphology, both relapsing with L2 blasts. These data do not support previous findings of a high frequency of conversion from L1 to L2 at relapse. They suggest, instead, that L2 morphology is not a common morphologic expression of initial treatment fa