Objective: We sought to determine whether paternal size at birth and during young adulthood influences the birth weight of the offspring. Study Design: This historic cohort study followed up girls born in Copenhagen during 1959 to 1961. Their pregnancies in 1974 to 1989 were traced through the Danish Population Register, and the Personal Identification Numbers of the fathers of the children were obtained. Paternal birth weight was obtained from midwifery records and adult stature from military draft records. Results: Compared with fathers who weighed at least 4 kg at birth, fathers who weighed 3 to 3.99 kg at birth had infants who were 109 gm lighter, and fathers who weighed <3 kg had infants who were 176 gm lighter after adjustment for maternal birth weight and adult stature, smoking, and medical and socioeconomic factors. After adjustment, fathers in the lowest quartile of adult body mass index had infants that were 105 gm lighter than those of fathers in the highest quartile. Both paternal birth weight and adult body mass index exhibited significant trends in association with infant birth weight. Conclusion: Independently of maternal size, the father's physical stature, particularly his own size at birth, influences the birth weight of his children. (Am J Obstet Gynecol 1998;178:1022-6.)