Physiologic, biochemical and genetic characteristcs of current smokers, non-smokers and ex-smokers were compared among a study population of almost 10,000 working men aged 40-65 years in Israel. Many significant differences were found, although most differences were small. Current and ex-smokers were slightly taller than non-smokers, and height was correlated to amount smoked. Current smokers had lower weights (absolute and adjusted)8 skunfolds, systolic and distolic blood pressures, and uric acid levels, but had higher pulse rates and higher concentrations of total and beta-cholesterol, hemoglobuin and hematocrit. They showes, however, lower alpha-cholesterol (absolute and relative) concentrations. All the above differences were dose-response-related but discriminant analysis incorporating socio-demographic and behavioral correlates revealed only a moderate degree of combined discrimination. The conclusion is that, while cigarette smokers differ from non-smokers in many ways, the overall difference is umimpressive. This explains why, in prospective analysis, increased mortality and morbidity among smokers could not be attributed to confounding variables. Ex-smokers resembled current smokers in height and cholesterol but were closer to non-smokers levels of weight, skinfolds, blood pressure, uric acid and alpha-cholesterol. Ex-smokers also had inermediate levels of hemoglobin and hematocrit. Cigar and pipe smokers differed from non-smokers only in their elevated levels of cholesterol and uric acid. There was no variation by smoking habits among blood groups of seven types. It is concluded that an overall similarity exists between different smoking groups mainly in terms of coronary risk factors, and that therefore, constitutional differences do no account for the increased mortality and morbidity among smokers.