CONTEXT: Low bone mineral density (BMD) is a serious public health problem. Osteoporotic fractures are associated with low bone mass, occurring frequently in the hip and spine. Previous studies have demonstrated a positive relationship between BMD and weightbearing exercise but not a similar positive relationship with nonweightbearing exercise. There is concern that cycling, a weight-supported sport, does not benefit bone health. OBJECTIVE: To systematically review the evidence suggesting that cyclists have impaired bone health at the femoral neck and lumbar spine. DATA SOURCES: Articles in PubMed, Cochrane Library, and CINAHL were identified in December 2009 based on the following terms and combinations: bicycling, bone density, cyclist. STUDY SELECTION: Thirteen studies satisfied inclusion criteria: 2 prospective studies (level of evidence 2b) and 11 cross-sectional studies (level of evidence 2c). DATA EXTRACTION: Data included sample size, demographics, description of cycling and control criteria, and BMD (g/cm(2)) at the lumbar spine, femoral neck, and hip. RESULTS: Two prospective studies showed a decrease in femoral neck, total hip, or lumbar spine BMD in cyclists over the study period. Four cross-sectional studies compared cyclists with sedentary controls, and 3 found cyclists' lumbar spine and femoral neck BMD similar to that of controls, whereas 1 found cyclists' BMD to be lower than that of controls. Seven cross-sectional studies compared cyclists with active controls: 2 found no differences in femoral neck and lumbar spine BMD between cyclists and controls; 4 found that cyclists had lower lumbar spine BMD than did active controls, including runners; and 1 reported a trend toward lower lumbar spine BMD in cyclists versus controls. CONCLUSIONS: There is concerning but inconsistent, limited-quality disease-oriented evidence-primarily from cross-sectional data-indicating that cyclists may be at risk for low bone mass, particularly at the lumbar spine. Additional longitudinal controlled intervention trials are needed.