49, XXXXY is a rare aneuploidy and variant of Klinefelter syndrome, occurring in 1 per 80,000–100,000 live births. We present a cohort of 40 affected males, focusing on musculoskeletal problems. Subjects were participants in an annual 49er family support group meeting. Children were examined in a multidisciplinary clinic by a pediatric neurologist and geneticist, a pediatric orthopedist, a neurodevelopmentalist, and two physical therapists. The patient data were collected from this clinic from 2004 to 2012. All patients were required to have karyotypes that confirmed the presence of XXXXY. There was a high prevalence of musculoskeletal disorders, particularly hypotonia (34 patients [85%]), radioulnar synostosis (30 [75%]), pes planus (26 [65%]), asymmetric hip rotation (27 [67.5%]), and clinodactyly (24 [60%]). Other, less common lower-extremity disorders, included, 5 patients (12.5%) with unilateral club foot, 5 boys (12.5%) with pes cavus, 10 patients (25%) genu valgum and 2 children with genu varus (5%). To our knowledge, this is the first large cohort of boys with 49, XXXXY that focuses on musculoskeletal disorders. There was an increased incidence of hypotonia, clubfoot, avascular necrosis of the femoral head, radioulnar synostosis, and pes planus compared to the normative population. Boys with 49, XXXXY would benefit from multidisciplinary evaluations, particularly from pediatric orthopedists, physical therapists, neurologists, and geneticists for appropriate medical care. © 2013 Wiley Periodicals, Inc.