Objectives:Studies on the association between parental age and bipolardisorder (BPD) are scarce and with inconsistent findings. The aim of thisstudy was to examine the association of parental age and age differencebetween parents with risk of BPD in offspring.Methods:This nested case–control study identified 1,861 cases ofindividuals with BPD born in Finland during 1983–1998 and diagnosedby the end of 2008, and 3,643 sex- and date of birth-matched controlsfrom nationwide population-based registers. Conditional logisticregression was used to examine the association adjusting for potentialconfounding due to age of the other parent, parental psychiatric history,educational level, and place of birth.Results:A U-shaped association of unadjusted odds ratios (ORs) forBPD risk was seen in different paternal age categories, with the oddsincreasing at both ends of the age spectrum. In the adjusted analyses,offspring of fathers aged≥50 years had a 2.8-fold increased odds[OR=2.84, 95% confidence interval (CI): 1.32–6.12] of BPD ascompared to those with fathers aged 30–34 years. The odds wereincreased 1.3-fold (OR=1.35, CI: 1.06–1.72) in fathers aged 20–24 years. No significant association was found between maternal ageand BPD in the adjusted analyses. Age difference between parents wasnot associated with BPD.Conclusions:The increased risk of BPD in offspring of the youngest andoldest fathers in the study suggests the involvement of differentbiological and psychosocial factors at the two ends of the paternal agespectrum. These findings may be significant in the context of advancingparental age in recent times.