Aging-induced elevation in C1q secretion activates the Wnt signaling pathway in muscles, leading to the development of muscle fibrosis. However, the association between serum C1q level and muscle mass and strength remains unclear in humans. The aim of the study was to elucidate whether serum C1q level is associated with aging- and resistance training-induced changes in muscle mass and strength. First, in a cross-sectional study, we investigated the association between serum C1q level and muscle mass and strength in 131 healthy subjects, aged 20-81 yr. Second, in an intervention study, we examined the association between the effects of serum C1q level and muscle mass and strength on 12 wk resistance training in 11 healthy older adults (60-81 yr). In the cross-sectional study, serum C1q level increased with aging and was negatively correlated with muscle mass and strength. Furthermore, 12 wk resistance training in older adults reduced the age-associated elevation in serum C1q levels. The training effect of serum C1q level significantly correlated with the change in the cross-sectional area of the thigh (r = -0.703; P < 0.01). Serum C1q level may reflect loss of muscle mass; therefore, C1q may be a novel biomarker of sarcopenia.