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The purpose of this study was to investigate the potential contribution of environmental factors and testosterone on male alopecia.Ninety-two identical male twins were recruited from 2009 to 2011. A comprehensive questionnaire was completed followed by the acquisition of sputum samples for testosterone analysis and standardized digital photography. Frontal, temporal, and vertex hair loss was assessed from these photographs. Hair loss was then correlated with survey responses and testosterone levels between twin pairs. Two independent, blinded observers also rated the photographs for hair thinning.Increased smoking duration (p < 0.001) and the presence of dandruff (p = 0.028) were significantly associated with increased frontal hair loss. Increased exercise duration (p = 0.002), consumption of more than four alcoholic drinks per week (p = 0.042), and increased money spent on hair loss products (p = 0.050) were all associated with increased temporal hair loss. Daily hat use (p = 0.050), higher body mass index (p = 0.012), and higher testosterone levels (p = 0.040) were associated with decreased temporal hair loss. Factors that were significantly associated with increased vertex hair loss included abstinence from alcohol consumption (p = 0.030), consumption of more than four alcoholic drinks per week (p = 0.004), increased smoking duration (p = 0.047), increased exercise duration (p = 0.050), and increased stress duration (p = 0.010). Lower body mass index, more children, increased caffeine consumption, history of skin disease, and abstinence from alcohol were significantly associated with increased hair thinning scores (p < 0.05).This study offers substantial evidence that exogenous factors may have a clinically significant impact on hair loss.Risk, III.