The Pediatric Oncology Group analyzed 103 cases of childhood acute lymphocytic leukemia (ALL) with an acid phosphatase stain and with a series of immunologic markers. As reported by others, the authors demonstrated a high correlation of acid phosphatase (AP) positivity and T-ALL. However, a subset of T-ALL was acid phosphatase negative, and some non-T, non-B, non-pre-B-ALL cases were AP positive. The predictive value of the AP test was, therefore, poor as a marker of T-ALL. AP-negative T-ALL cases appeared to be a distinctive subset of T-ALL, and AP negativity an intrinsic characteristic of this subset, rather than a failure of the test system. AP-positive n-ALL cases demonstrated no difference from AP-negative cases and, in particular, no evidence of early T-ALL differentiation.