Eighteen patients referred to The Johns Hopkins Oncology Center for treatment of acute myelocytic leukemia during a 2-year period had pretreatment wbc counts greater than 50,000/microliter. Despite immediate admission, aggressive supportive care, and rapid institution of chemotherapy, six patients died from complications of the high wbc count at presentation. The patients who died had significantly higher initial wbc counts, lower platelet counts, and lower arterial oxygen saturation; they were older, more likely to be female, and more likely to have abnormal serum urea nitrogen than the 12 patients who survived. Five of the six patients who died had experienced a delay of greater than 48 hours from diagnosis to referral, and only one of the 12 patients who survived was not promptly referred for treatment at the time of diagnosis (P = 0.004). A mathematical model, the logit of the fraction of the initial wbc count versus the logarithm of time after starting therapy, was used to describe the response of the wbc count to chemotherapy. This logit analysis reduced the data for individual patients to a linear function, permitting statistical description of the population as a whole. This statistical description makes possible the comparison of different approaches to lowering the wbc count in such patients and the correlation of rate of response of wbc count with the probability of achieving remission and with duration of remission.