Alcohol abuse is usually regarded as the most likely cause of elevated serum liver enzyme values in those attending for well population screening, but we have found increased body weight to be an important contributing factor. We have measured serum levels of alanine amino-transferase (ALT), aspartate aminotransferase (AST) and gamma-glutamyl transferase (GGT) in approximately 21,000 men attending for routine health screening, and related these to behavioural factors such as alcohol consumption, cigarette smoking, exercise level and obesity. The levels of all three enzymes were positively correlated with levels of alcohol consumption. Decreasing levels of physical activity were associated with increases in mean ALT and GGT levels. Cigarette smoking showed only a weak effect on ALT and AST, which became non-significant after multivariate statistical analysis, but increasing consumption of cigarettes was associated with increased mean levels of GGT. In contrast, all three enzymes showed marked increases in mean levels with increasing body mass index (BMI). The effect of obesity was particularly important in the case of ALT: the prevalence of increased ALT values in obese subjects (BMI greater than or equal to 31 kg/m2) was more than eight times that in those with normal weight (BMI less than or equal to 25 kg/m2), even after allowing for the confounding effect of alcohol consumption. This study is concerned solely with male subjects, but we hope to extend the analysis to females in the near future.