We evaluated prospectively the incidence and risk factors of the metabolic syndrome (MS) and its components in 170 adult patients (mean age at evaluation: 24.8±5.4 years) who received an hematopoietic stem cell transplantation for childhood ALL, n=119, or AML, n=51. TBI was carried out in 124 cases; a busulfan-based conditioning was done in 30 patients. Twenty-nine patients developed a MS (17.1%, 95% confidence intervals: 11.7-23.6). The cumulative incidence was 13.4% at 25 years of age and 35.5% at 35 years of age. A higher body mass index (BMI) before transplantation and a growth hormone deficiency were associated with increased MS risk (P=0.002 and 0.01, respectively). MS risk was similar for patients who received TBI or busulfan-based conditioning. The TBI use increased the hyperglycemia risk (odds ratio (OR): 4.7, P=0.02). Women were at the risk of developing increased waist circumference (OR: 7.18, P=0.003) and low levels of high-density lipoprotein cholesterol (OR: 2.72, P=0.007). The steroid dose was not a risk factor. The MS occurs frequently among transplanted survivors of childhood leukemia. Its incidence increases with age. Both intrinsic (BMI, gender) and extrinsic factors (TBI, alkylating agents) contribute to its etiopathogenesis.