The prognostic significance of a left shift in the peripheral blood at the time of diagnosis of acute lymphocytic leukemia was investigated by a retrospective analysis of 109 patients treated on the same protocol in a single institution. Left shift was defined as the presence of 1% or more of metamyelocytes, myelocytes, or promyelocytes. All peripheral blood films were checked at the time of diagnosis by one of the authors. It was found that the duration of complete remission at 92 mo was 74% in patients with left shift and 42% in those without left shift (p less than 0.05, log-rank test). By Cox regression analysis, only the total white cell count (p less than 0.001) and the presence or absence of left shift (p less than 0.01) were independently significant in determining the proportion of patients in complete remission. Patients with a left shift had a significantly higher granulocyte count at diagnosis (p less than 0.05). We postulate that left shift in the peripheral blood count at the time of diagnosis may be an indirect measure of the total leukemia cell load. It is a new prognostic factor of significance in determining the likely outcome of the disease.