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There is growing evidence that advanced paternal age can be a reproductive hazard.We studied couples and their first children using nationwide registers in Denmark between 1980 and 1996. We restricted the analysis to mother's age 20–29 years. We estimated odds ratios (ORs) of preterm (<37 weeks gestation) and very preterm birth (<32 weeks) as a function of paternal age using logistic regression to adjust for potential confounding variables.The risk of preterm birth increased with paternal age, almost entirely resulting from an association for very preterm birth. Compared with fathers age 20–24 years, ORs for very preterm birth were 1.3 (age 25–29), 1.4 (age 35–39), 1.7 (age 40–44), 1.6 (age 45–49), and 2.1 (age 50+) (test for trend: P = 0.01).Risk of very preterm birth increases among older fathers, perhaps as a result of a paternal placental effect.