Objective: The purpose of this study was to determinewhether or not imatinib mesylate therapy induces growth hormone deficiency (GHD). Subjects and Methods: Seventeen patients with chronic myloid leukemia (CML) were enrolled in the study. The glucagon stimulation test (GST), and standard deviation scores (SDSs) of insulin-like growth fac- tor 1 (IGF-I) and insulin-like growth factor binding protein (IGFBP-3) were used to determine GHD. The L-dopa test was performed on those with IGF-I SDSs above the –1.8 cut-off level. Results: Of the 17 patients in the study, 12 (70%) had severe GHD (serum GH level <3 μg/l after GST). IGF-I SDSs and IGFBP-3 SDSs were below –1.8 in 12 patients (70%) and below –0.9 in 10 subjects (58%). Four of the 5 remaining subjects with IGF-I SDS >–1.8 showed insufficient GH response to L-dopa stimulation. Nine subjects (52%) had both severe GHD based on GST response and IGF-I SDS below –1.8. If an IGF-I SDS cut-off value l<–3 were used,5 out of 17 subjects (30%) would be classified as GH deficient. These same patients also showed severe GHD based on GST response. Conclusions: The data showed that a large number of patients on imatinib mesylate therapy had GH deficiency. A study involving a larger number of patients with a matched control group is needed to confirm the present observations.