The effect of smoking on the presence of antinuclear antibodies and on the occurrence of mitochondrial changes in lymphocytes was studied in 518 subjects over 45 years of age without overt diseases. Antinuclear antibodies could be demonstrated more frequently in smokers than in non-smokers. Smoking did not influence the age dependent degenerative changes in the mitochondria of the lymphocytes, but giant mitochondria could be seen more frequently in smokers than in non-smokers.