Differences in skeletal muscle characteristics between smokers and non-smokers have been demonstrated in a previous study ( Orlander , J. et al. 1979, Acta Physiol Scand 107:39-46). In order to decide whether these differences had a genetical background, six pairs of smoking-discordant monozygous twins were studied with respect to muscle (vastus lateralis) morphology, metabolism and function. The percentage type I fibres was lower in the smokers, who also had smaller diameters of this fibre type. Cytochrome oxidase activity was decreased in the smokers. No differences were seen for other enzymes of energy metabolism, capillary density, isometric or dynamic strength, or short-term muscular endurance. The non-smokers tended to be more physically active. In four ex-smoker/non-smoker pairs, no significant differences were found for the investigated parameters. It was concluded, that the difference in fibre type distribution is not due to a hereditary predisposition to take up smoking. Furthermore, the small difference in physical activity level is an unlikely cause. Thus, smoking per se appears to be the most plausible explanation for the difference in fibre type distribution and associated muscle characteristics. No definitive conclusion regarding the reversibility of the smoking-related differences can be drawn from the present results.