Objectives: Increased oxidative stress generated nucleoside damage seems to play a crucial role in bipolar disorder (BD) pathophysiology. It may contribute to accelerated ageing and reduced life expectancy in patients with BD. Methods: In the five-year prospective “Bipolar Illness Onset study”, we investigated repeated measurements of oxidative stress generated RNA and DNA damage in 357 patients with newly diagnosed/first-episode BD (880 visits), 132 of their unaffected first-degree relatives (236 visits) and 198 healthy age- and sex-matched control persons with no personal or first-degree family history of affective disorder (432 visits). Amongst patients with BD, we further investigated associations of oxidative stress generated RNA- and DNA damage with affective phases and measures of illness load. Results: Patients newly diagnosed with BD and their unaffected relatives had higher levels of oxidative stress generated RNA damage than healthy control individuals and these differences persisted over time, whereas DNA damage was less consistently elevated. Neither illness load nor affective phase impacted the levels in patients with BD. Conclusions: Our findings support elevated oxidative stress generated RNA damage being a trait phenomenon in BD as indicated by persistent increase in RNA damage over time in patients newly diagnosed with BD and in their unaffected first-degree relatives compared with healthy control individuals. We did not detect state alterations in levels of oxidative stress.