Diagnostic bone marrow smears from 132 patients with acute lymphoblastic leukaemia, (ALL) were stained simultaneously by the periodic acid-Schiff (PAS) reaction, and the blast cell positivity was assessed quantitatively. The patients fell naturally into two unequal groups: those with more than 20% PAS-positive blast cells (44 patients) and those with less (88 patients). There was no relation between the degree of positivity and age, sex, or presenting leucocyte count. Actuarial survival studies showed that the group with more than 20% PAS-positive blast cells survived longer, but that this difference assumed statistical significance only after the exclusion of patients over 14 years old and those with high white cell counts at the time of diagnosis. It appears that the PAS reaction can identify long survivors among patients with ALL, but not in the absence of features strongly associated with a poor prognosis.