['Low IGF-I signaling activity prolongs lifespan in certain animal models, but the precise role of IGF-I in human survival remains controversial. The IGF-I kinase receptor activation assay is a novel method for measuring IGF-I bioactivity in human serum. We speculated that determination of circulating IGF-I bioactivity is more informative than levels of immunoreactive IGF-I.', 'Our objective was to study IGF-I bioactivity in relation to human survival. DESIGN, SETTING, AND STUDY PARTICIPANTS: We conducted a prospective observational study at a clinical research center at a university hospital of 376 healthy elderly men (aged 73-94 yr).', 'IGF-I bioactivity was determined by the IGF-I kinase receptor activation assay. Total and free IGF-I were determined by IGF-I immunoassays. Mortality was registered during follow-up (mean 82 months).', 'During the follow-up period of 8.6 yr, 170 men (45%) died. Survival of subjects in the highest quartile of IGF-I bioactivity was significantly better than in the lowest quartile, both in the total study group [hazard ratio (HR) = 1.8; 95% confidence interval (95% CI) = 1.2-2.8; P = 0.01] as well as in subgroups having a medical history of cardiovascular disease (HR = 2.4; 95% CI = 1.3-4.3; P = 0.003) or a high inflammatory risk profile (HR = 2.3; 95% CI = 1.2-4.5; P = 0.01). Significant relationships were not observed for total or free IGF-I.'] Do low circulating insulin-like growth factor I bioactivity in elderly men is associated with increased mortality?