Essentially all cases of precursor B-lymphoblastic leukemia/lymphoma (B-ALL) demonstrate multiple immunophenotypic aberrancies relative to normal maturing B-cell precursors (hematogones). The stability of these aberrancies has relevance to follow-up minimal residual disease analysis. We compared the immunophenotypes at diagnosis and relapse in 51 childhood and adult B-ALLs with flow cytometry (FC) using broad antibody panels. A total of 446 aberrancies were present at diagnosis (median, 9 per case; range, 2-14). All cases retained multiple aberrancies at relapse (median, 8 per case; range, 2-14). Antibody panels at relapse allowed assessment of 383 (85.9%) of the initial 446 aberrancies. Of these, 299 (78.1%) were persistent and 84 (21.9%) were lost at relapse. Overall, 73% of cases showed a loss of at least 1 aberrancy at relapse. However, new aberrancies were detected in 60% of cases. These findings suggest that FC is suitable for the detection of residual B-ALL, provided that follow-up studies are not too narrowly targeted.