6598 Background: In a multicenter placebo-controlled study, LEN (10 mg/day for 1–21 days or 5 mg/day for 1–28 days; 28-day cycle) achieved significant RBC transfusion independence for ≥ 26 wks (56% and 41%) and cytogenetic responses (41% and 17%) in RBC transfusion-dependent patients (pts) with Low-/Int-1-risk MDS with del(5q). The most common grade (G) 3–4 adverse events (AEs) were neutropenia and thrombocytopenia. Here, we describe the frequency, timing and management of LEN-associated hematologic AEs. Methods: AEs (NCI CTCAE v3) occurring during the 1 y double-blind phase are summarized. Dose reductions were required for G4 neutropenia or thrombocytopenia by protocol; G-CSF/GM-CSF use was allowed. Results: All 138 pts randomized to LEN (10 mg = 69, 5 mg = 69) who received ≥ 1 dose were included. Median (range) exposure duration in wks was 50 (1–56) for LEN 10 mg and 18 (2–53) for LEN 5 mg (higher response rate with 10 mg explaining longer exposure). In the LEN 10 mg group G3–4 neutropenia was most comm...