In the population-based Finn Twin 16 study, proportions of genetic and environmental factors contributing to muscle dissatisfaction and muscle-enhancing substance use were assessed in 319 pairs of twin brothers: 141 monozygotic (MZ) and 178 dizygotic (DZ) pairs. In addition there were 86 twin individuals from pairs in which only one co-twin responded. Of all respondents, 30% experienced high muscle dissatisfaction. The corresponding proportion of muscle-enhancing substance use was 10%. The subjects were similar in age (23.8 years, 95% confidence interval [CI] 23.76-23.84), body mass index (23.7, 95% CI 23.5-23.9), and waist circumference (84.5 cm, 95% CI 83.7-85.2), independent of their muscle dissatisfaction or muscle-enhancing substance use status and independent of their zygosity. The MZ polychoric correlation for muscle dissatisfaction was .39 (95% CI .17-.58) and .27 for DZ pairs (95% CI .07-.46). The MZ tetrachoric correlation for muscle-enhancing substance use was .65 (95% CI .28-.87) and .56 for DZ pairs (95% CI .26-.78). The AE model, where additive genetic factors (A) accounted for 42% (95% CI .23-.59) and unique environmental factors (E) 58% (95% CI .41-.77) of the liability, provided the best fit for muscle dissatisfaction. The CE model, where common environmental factors (C) accounted for 60% (95% CI .37-.77) and unique environmental factors (E) 40% (95% CI .23-.63) of the liability, provided the best fit for muscle-enhancing substance use. Both genetic and unique (nonfamilial) environmental factors are involved in muscle dissatisfaction in the population. Nongenetic factors (both familial and nonfamilial) appear to best explain the use of muscle-enhancing substances.