Cytarabine (Ara-C) is currently used in the treatment of adult acute myeloid leukemia (AML). To predict the results of induction chemotherapy, it could be useful to detect leukemic cells that are resistant to Ara-C in patients with AML. Using a bromodeoxyuridine/DNA (BrdUrd/DNA) staining method in flow cytometry (FCM), we have developed a cell resistance index to Ara-C (RI). The technique has been applied to 121 bone marrow (BM) samples from patients with de novo AML treated by a regimen containing Ara-C and daunorubicin (DNR). Ninety-seven patients achieved a complete remission (CR), and 24 patients did not and were considered drug-resistant (DR). The BM cells collected at diagnosis were cultured for 48 hours and underwent BrdUrd/DNA analysis. Among 25 patients with no or very low proliferative activity (<3% of cells in S-phase), the proportion of DR patients (nine of 25) was significantly higher than in a second group of 96 patients with detectable proliferative activity (15 of 96) (P < .025). Within this second group, there was a first group of nine patients with high RI values, which included only DR patients; a second group of 63 patients with low RI values, which included 62 CR patients; and a third group of 24 patients with intermediate RI values, which included 19 CR and five DR patients. In view of this series, our results show that it is possible to detect a majority of DR patients treated by Ara-C.