Preterm birth is the leading cause of infant mortality in developed countries, but the association between gestational age at birth and mortality in adulthood remains unknown. To examine the association between gestational age at birth and mortality in young adulthood. National cohort study of 674,820 individuals born as singletons in Sweden in 1973 through 1979 who survived to age 1 year, including 27,979 born preterm (gestational age <37 weeks), followed up to 2008 (ages 29-36 years). All-cause and cause-specific mortality. A total of 7095 deaths occurred in 20.8 million person-years of follow-up. Among individuals still alive at the beginning of each age range, a strong inverse association was found between gestational age at birth and mortality in early childhood (ages 1-5 years: adjusted hazard ratio [aHR] for each additional week of gestation, 0.92; 95% CI, 0.89-0.94; P < .001), which disappeared in late childhood (ages 6-12 years: aHR, 0.99; 95% CI, 0.95-1.03; P = .61) and adolescence (ages 13-17 years: aHR, 0.99; 95% CI, 0.95-1.03; P = .64) and then reappeared in young adulthood (ages 18-36 years: aHR, 0.96; 95% CI, 0.94-0.97; P < .001). In young adulthood, mortality rates (per 1000 person-years) by gestational age at birth were 0.94 for 22 to 27 weeks, 0.86 for 28 to 33 weeks, 0.65 for 34 to 36 weeks, 0.46 for 37 to 42 weeks (full-term), and 0.54 for 43 or more weeks. Preterm birth was associated with increased mortality in young adulthood even among individuals born late preterm (34-36 weeks, aHR, 1.31; 95% CI, 1.13-1.50; P < .001), relative to those born full-term. In young adulthood, gestational age at birth had the strongest inverse association with mortality from congenital anomalies and respiratory, endocrine, and cardiovascular disorders and was not associated with mortality from neurological disorders, cancer, or injury. After excluding earlier deaths, low gestational age at birth was independently associated with increased mortality in early childhood and young adulthood.