Blast cell morphology was assessed at diagnosis and subsequent bone marrow relapse in 33 unselected patients with lymphoblastic leukemia (ALL). Each marrow was classified 'blind' according to the French-American-British (FAB) criteria, and it was found that 19 of 24 (79%) patients initially typed as FAB L1 changed to FAB L2 during the course of their disease, but no patient made the reverse morphological change (p is less than 0.001). Five patients retained FAB L1 appearances; these included three of the four who had T-cell markers. One patient typed as FAB L3 did so consistently. This study indicates that FAB L2 ALL frequently emerges as a treatment-resistant offshoot of FAB L1 and provides further evidence that this marks a more aggressive form of the disease.
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