All-trans retinoic acid (ATRA) is used in the treatment of acute promyelocytic leukemia. Because ATRA has effects (increase in apoptosis, suppression of bcl-2), it has also been,used for the treatment of other French-American-British (FAB) subtypes of acute myelogenous leukemia (AML). To find out the in vivo and in vitro effects of ATRA in AML, we analyzed 37 patients with de novo AML. Twenty-seven patients received ATRA before remission-induction (RI) treatment (ATRA Group). Results were compared to a control Group (10 patients) that received induction without ATRA during the same time period. Bone marrow or peripheral blood samples were collected from all patients on d 0 and 4. The immunphenotype, myeloperoxidase (MPO), reaction and the efflux uptake of rhodamine 123 (Rbl23) were analyzed on myeloblasts in these samples. In the myeloblasts from patients treated with ATRA, the uptake of Rh123 was increased significantly (p = 0.026) from d 0 to d 4, and all other parameters remained unaltered. ATRA administration increased the complete remission (CR) rate (88%, 22/25 vs 55%, 5/9) significantly (p = 0.042). Logistic regression analysis revealed that ATRA administration was the important factor in CR, among other potential factors including age, white blood count, bcl-2 expression, and the uptake and efflux of Rh 123 (p = 0.05). Estimated disease-free survival and overall survival were similar between these two Groups (43% vs 37.5% and 51.2% vs 37.5%, respectively). In conclusion, ATRA treatment prior to RI treatment may improve the CR rate in patients with de novo AML, which seems to be related to its beneficial effect on multidrug resistance.