We investigated the effectiveness and tolerability of azacitidine in patients with World Health Organization-defined myelodysplastic syndromes, or acute myeloid leukemia with 20-30% bone marrow blasts. Patients were treated with azacitidine, with one of three dosage regimens: for 5 days (AZA 5); 7 days including a 2-day break (AZA 5-2-2); or 7 days (AZA 7); all 28-day cycles. Overall response rates were 39.4%, 67.9%, and 51.3%, respectively, and median overall survival (OS) durations were 13.2, 19.1, and 14.9 months. Neutropenia was the most common grade 3-4 adverse event. These results suggest better effectiveness-tolerability profiles for 7-day schedules.