Phase I and II clinical studies demonstrated activity of Clofarabine in acute leukemias. In previous studies we have investigated clofarabine, plus ara-C combinations and reported a CR rate of 24% in relapsed AML and 52% in previously untreated AML ≥ 50 years (yrs) with acceptable toxicity profile. Anthracyclines are active in AML. To explore clofarabine further in AML combinations we conducted a phase I study of clofarabine with idarubicin with or without ara-C in pts with relapsed AML, MDS, and CML. Considered as dose-limiting toxicities (DLT) are ≥ grade 3 drug-related toxicities. Maximum tolerated dose (MTD) will be determined by “3+3” dose escalation scheme. On the clofarabine (C)/idarubicin (I) combination (CI), 9 AML pts are enrolled (2 primary refractory, 7 first relapse). Median age: 58 yrs (range 24–71). Median first remission duration (CRD1): 3.1 mos. (0–7.6). For the first dose level, C was given at 22.5mg/m2 i.v. daily x 5d and I at 12mg/m2 i.v. daily x 3d. Among the first 6 pts, 2 ≥ gr. 3 toxicities (diarrhea, rash, ↑ bili) occurred necessitating dose de-escalation of C to 15mg/m2 i.v. daily x 5 and I 8mg/m2 i.v. daily x 3. Among 3 pts, 1 ≥ gr.3 toxicity (↑ bili) was observed. No responses occurred. On the CI + ara-C arm (CIA), 7 AML pts are enrolled (1 primary refractory, 6 first relapse). Median age: 58 yrs. (24–78). Median CRD1: 11.2 mos. (0–13.1). First dose level: C 22.5mg/m2 i.v. daily x 5d, I 8mg/m2 i.v. daily x 3d, A 1g/m2 i.v. daily x 5d. Of 3 pts, 2 developed ≥ gr.3 toxicities (↑ bili, diarrhea) leading to the following de-escalation: C 15mg/m2 i.v. daily x 5d, I 6mg/m2 i.v. daily x 3d, A 0.75g/m2 i.v. daily x 5d. Of 4 pts (1 ≥ gr. 3 rash, ↑ bili), 3 pts achieved CR. The phase I study is ongoing until determination of DLT and MTD for each arm. Our preliminary results indicate clinical activity of CIA even at the low dose level.