ABSTRACTThe leukemic cells of 50 children with nullcell acute lymphoblastic leukemia (null‐ALL) were studied at the time of diagnosis for the presence or absence of an ALL cell‐associated antigen (ALLA). Eleven of the patients were ALLA (−) and the remainder were ALLA(+). The clinical courses of these patients were followed prospectively for 2‐34 months. In the ALLA(−) group, 7 of 11 patients suffered a disease‐related adverse event [induction failure; bone marrow (BM), central nervous system (CNS) or testicular relapse; death] by 13 months from diagnosis. In the ALLA (+) group, there were seven disease‐related adverse events in 39 patients within 29 months after diagnosis. The difference in the incidence of diseaserelated adverse events between the two groups is statistically significant, i.e., 7/11 versus 7/39 (p= 0.005 by Fisher's Exact Test). The results suggest that children with ALLA(−) null‐ALL have a poorer prognosis than those who are ALLA(+).