BACKGROUND: This study examined differences in ADHD symptoms and diagnosis between preterm and term-born adults (≥18 years), and tested if ADHD is related to gestational age, birth weight, multiple births, or neonatal complications in preterm borns. METHODS: (1) A systematic review compared ADHD symptom self-reports and diagnosis between preterm and term-born adults published in PubMed, Web of Science, and PROQUEST until April 2021; (2) a one-stage Individual Participant Data(IPD) meta-analysis (n = 1385 preterm, n = 1633 term; born 1978-1995) examined differences in self-reported ADHD symptoms[age 18-36 years]; and (3) a population-based register-linkage study of all live births in Finland (01/01/1987-31/12/1998; n = 37538 preterm, n = 691,616 term) examined ADHD diagnosis risk in adulthood (≥18 years) until 31/12/2016. RESULTS: Systematic review results were conflicting. In the IPD meta-analysis, ADHD symptoms levels were similar across groups (mean z-score difference 0.00;95% confidence interval [95% CI] -0.07, 0.07). Whereas in the register-linkage study, adults born preterm had a higher relative risk (RR) for ADHD diagnosis compared to term controls (RR = 1.26, 95% CI 1.12, 1.41, p < 0.001). Among preterms, as gestation length (RR = 0.93, 95% CI 0.89, 0.97, p < 0.001) and SD birth weight z-score (RR = 0.88, 95% CI 0.80, 0.97, p < 0.001) increased, ADHD risk decreased. CONCLUSIONS: While preterm adults may not report higher levels of ADHD symptoms, their risk of ADHD diagnosis in adulthood is higher. IMPACT: Preterm-born adults do not self-report higher levels of ADHD symptoms, yet are more likely to receive an ADHD diagnosis in adulthood compared to term-borns. Previous evidence has consisted of limited sample sizes of adults and used different methods with inconsistent findings. This study assessed adult self-reported symptoms across 8 harmonized cohorts and contrasted the findings with diagnosed ADHD in a population-based register-linkage study. Preterm-born adults may not self-report increased ADHD symptoms. However, they have a higher risk of ADHD diagnosis, warranting preventive strategies and interventions to reduce the presentation of more severe ADHD symptomatology in adulthood.