In 55 placentae, collected at term from clinically normal pregnancies, hydroxyproline was assayed. Thirty were from mothers who smoke and 25 from nonsmokers. In placentae of smokers, hydroxyproline concentration was 42.77 +/- 8.77 micrograms/mg of tissue and in these of nonsmokers 22.52 +/- 7.46 micrograms/mg. Such high a difference discloses a large increment in placental collagen production in smokers, which may account for some of the noxious effects of tobacco on pregnancy.