Chronic myelogenous leukemia (CML) is caused by the BCR-ABL1 fusion gene that encodes for a constitutively-active tyrosine kinase. Adults and children with CML are typically treated with imatinib mesylate, a BCR-ABL1 tyrosine kinase inhibitor (TKI), or a second-generation TKI. Several case reports have documented growth delay of unknown mechanism in children with CML treated with imatinib. We report a seven-year-old identical twin with CML who developed significant growth delay, as compared to her twin, during five years of TKI therapy. Detailed endocrine evaluation showed acquired growth hormone deficiency, a pathway potentially inhibited by TKIs.