Despite no chemotherapy and a marrow morphologically typical of frank relapse, an acute myelogenous leukemia (AML) patient survived for nearly 1 yr. During this time she remained asymptomatic and maintained nearly normal levels of platelets and hemoglobin. Cytochemical and electron microscopic studies of her bone marrow in liquid culture revealed on several occasions a unique maturational sequence in that leukemic cells differentiated to form morphologically abnormal primary granules which appeared to rupture and cause cytolysis of these cells. In these cultures, blasts rapidly disappeared and were replaced by more mature granulocytes, in contrast to observations in cultures derived from five other patients with AML in relapse which showed persistently elevated blast counts with no evidence of maturation in vitro. These findings support the concept that in AML cell maturation is regularly impaired and in some cases also aberrant. In addition, the abnormal granule formation with autolysis of the leukemic cells observed in one patient may explain both the early cell death in vitro and this patient's relatively indolent clinical course. Similar in vitro studies may help predict atypical clinical courses in patients with AML and facilitate design of appropriate chemotherapy.