Abstract Context: Myostatin is expressed mainly in skeletal muscle cells and acts as an inhibitor of muscle growth and differentiation. However, data on the determinants of serum myostatin concentrations in humans are limited. Objective: The aim of the study was to assess the correlates of serum myostatin concentrations in men. Design: We conducted a cross-sectional analysis of the STRAMBO cohort. Setting: Men holding private health insurance coverage with Mutuelle de Travailleurs de la Région Lyonnaise were included in the study. Participants: A total of 1121 male volunteers aged 20–87 yr participated in the study. Interventions: Nonfasting blood samples were collected. Main Outcome Measures: We measured the association of the investigated variables with circulating myostatin levels. Results: Serum myostatin levels increased slightly with age until 57 yr and then decreased. Circulating myostatin levels showed circannual variation, with the highest concentration in spring. In men older than 57 yr, serum myostatin levels decreased across increasing quartiles of body mass index and of total central and peripheral fat mass (P < 0.05 to < 0.001). Serum myostatin levels were positively correlated with serum levels of 25-hydroxycholecalciferol (25OHD), even after adjustment for season. Average myostatin levels were 0.47 sd higher in men with 25OHD above 40 ng/ml, compared with those with 25OHD below 20 ng/ml (P < 0.05). Current smokers had lower myostatin concentration. Neither current physical activity nor serum levels of PTH, testosterone, and 17β-estradiol were associated with myostatin concentrations. Conclusions: In men, circulating myostatin levels show seasonal changes and are associated with age, body mass index, fat mass, smoking, and 25OHD levels.