Background: The number of CAG repeats (CAGn) within the CAG repeat polymorphism of the androgen receptor gene correlates inversely with the transactivation of the receptor. Objective: To examine the impact of CAGn on muscle, fat distribution, and circulating androgen levels. Design, settings and participants: Population-based, cross-sectional study of 783 Danish men aged 20–29 years. Methods: Genotyping was performed in 767 men. Areas of thigh and lower trunk muscle (musclethigh and musclelower trunk), subcutaneous adipose tissues (SATthigh and SATlower trunk), and deep adipose tissues (i.m. and visceral) were measured in 393 men by magnetic resonance imaging (MRI). Lean body mass (LBM) and fat mass (FM) were measured in all men by whole body dual-energy X-ray absorptiometry (DEXA). The absolute areas acquired by MRI were the main outcomes. The absolute DEXA measurements and relative assessments of both modalities were considered as the secondary outcomes. Results: CAGn (range: 10–32) correlated inversely with absolute musclethigh (r=–0.108), absolute musclelower trunk (r=–0.132), relative musclethigh (r=–0.128), relative musclelower trunk (r=–0.126), relative LBMlower extremity (r=–0.108), and relative LBMtotal (r=–0.082), and positively with relative SATthigh (r=0.137), relative SATlower trunk (r=0.188), relative FMlower extremity (r=0.107), and relative FMtotal (r=0.082). These relationships remained significant, controlling for physical activity, smoking, chronic disease, and age. CAGn did not correlate with any circulating androgen. Conclusions: The CAG repeat polymorphism affects body composition in young men: absolute musclethigh and absolute musclelower trunk increase as CAGn decreases. Expressed relatively, muscle areas and LBM increase, while SAT and FM decrease as CAGn decreases. The polymorphism does not affect deep adipose tissues or circulating androgen levels in young men.