Serum cholesterol, serum glyceride, blood glucose, relative weight, physical activity, percent calories from fat, percent calories from carbohydrate, and polyunsaturated/saturated fatty acid (P/S) ratio were determined in 1780 rural and 4023 urban fasting men aged 45-64 years being followed prospectively to ascertain reasons for the low prevalence of coronary heart disease mortality in Puerto Rico as compared to other countries. Lipid and glucose levels and relative weight were higher and physical activity lower in urban men. Glyceride was positively correlated with cholesterol, glucose, and relative weight, and negatively correlated with physical activity. Cholesterol was positively correlated with glyceride, glucose, and relative weight, and not correlated with physical activity. Percent calories from fat and P/S ratio were significantly higher and percent carbohydrates from starch significantly lower in urban men. These variables taken together were not significant in predicting the differences between urban and rural serum cholesterol or glyceride values. Relative weight was the most significant variable in explaining urban-rural differences in serum glyceride and cholesterol levels. These findings confirm the importance of weight control in dietary management for reduction of serum lipids.