Viscosity values of marrow cells from patients suffering from acute promyelocytic leukemia (acute myeloid leukemia of type 3) have been determined before and during a treatment with all-trans retinoïc acid (active metabolite of the vitamin A, inducing cell differentiation and maturation). Evaluated from aspiration tests into a glass micropipette, the marrow cell viscosity has been compared to that of normal neutrophils. Results seem to point out that the induced hyperleucocytosis is favoured by both a low standard deviation of pathological cell viscosity and a cell viscosity value close to that of normal neutrophils. The systematic study of cell viscosity could therefore help the clinicians to individualize the treatment.