The cases of one hundred and forty-one patients (85 males, 56 females) treated for hyperleucocytic acute lymphocytic leukemia (H-ALL) were reviewed. In all cases the initial white blood cell count was over 100,000/cu mm. One hundred patients (71%) attained complete remission (CR). The median duration of CR was six months and the median survival was nine months for all patients and 11 months for those who attained CR. Age, initial hemoglobin, and the height of initial white blood cell count over 100,000 had no significant prognostic value. Relapses occurred earlier in patients with a mediastinal mass. The results depended on the treatment used. With modern treatment, including more intensive chemotherapy and central nervous system prophylaxis, CR rate increased from 65% to 81% and median duration of CR improved from four months to ten months. The most important prognostic difference was related to the sex: CR rate was higher (78.5% vs. 66%) and median duration of CR and hematological remission was longer for females (nine months vs. six months and ten months vs. 6.5 months, respectively). This difference only appeared with modern treatments, however: before 1972 the median duration of CR was four months for both sexes, and after 1972, it was eight months for males and 17 months for females. This difference could be explained by the site of the first relapse, which was testicular in only 2% of cases before 1972 and 27% (47% of the males who relapsed) after 1972.