Previous classification in vitro of adult acute leukemia incorporating morphology has been complex and difficult to understand. We have devised a simplified classification based solely on leuekemic proliferation in vitro. Seventy-six patients with adult acute leukemia previously untreated were included in this study and received identical chemotherapy. Three groups were recognized. The complete remission rate was 76% in the 21 patients with no leukemic growth in vitro (Group 1), 75% in 36 patients with leukemic cell growth but aggregates of 20 cells or less (Group 2), and only 21% in the 15 patients with aggregates of greater than 20 (Group 3). There was a highly significant difference in complete remission rates between Group 3 and the other two groups (p less than 0.001). Linear logistic regression analysis demonstrated the independence of the growth in vitro from other prognostic variables. A predictive model utilizing the in vitro result more accurately predicted for remission, both retrospectively and prospectively, than a model constructed with presently known prognostic parameters. The cause of death in failures suggested that this system detects resistance to the chemotherapy.