Umbilical arteries from 52 women, 22 heavy smokers (more than 10 cigarettes per day) and 30 non-smokers were studied by transmission electron microscopy. An increased number of mitochondrial profiles was observed in many endothelial cells of smokers: chi 2 = 16.49, P less than 0.0001. The mean number of mitochondrial profiles was 23 per endothelial cell compared with 3 in non-smokers. Mitochondrial volume density (area covered by mitochondrial profiles/area cytoplasm) was measured and found to range from 0.15 to 0.62 in smokers, and 0.01 to 0.02 in non-smokers. Mitochondrial index (mitochondrial volume density/number of mitochondrial profiles) was calculated and found to be 0.008-0.060, mean value 0.018, in smokers, and 0.003-0.008, mean value 0.006, in non-smokers. Thus, in smokers a larger area of the endothelial cell cytoplasm was often occupied by mitochondria, a greater number of mitochondrial profiles was found, but the mean size of the mitochondria (profiles) was only slightly increased.