Only few studies have assessed the role of physical activity in the etiology of ovarian cancer, and the results have been inconclusive. We studied associations between physical activity and risk of ovarian cancer in 96,541 women aged 30–49 at enrollment in a prospective study in Norway and Sweden. Participants reported physical activity level at ages 14, 30 and at enrollment, and participation in competitive sports. Complete follow-up through 2001/2002 was achieved by linkage to national registries. The relation between physical activity and ovarian cancer incidence was assessed using multivariate Cox proportional hazard models. During an average 11.1 years of follow-up, there were 264 ovarian cancer cases (including 81 borderline tumors) diagnosed at a mean age of 49 years. Highly physically active women at cohort enrollment had a similar risk of ovarian cancer as women reporting no activity (multivariate relative risk RR = 1.08, 95% CI 0.53–2.18). Physical activity at age 30 or at age 14 did not either afford any protection from ovarian cancer, nor did a consistently high level of activity from younger ages until enrollment. Results were similar for invasive and borderline tumors, and for different subgroups of women classified according to other known risk factors for ovarian cancer. In our study of primarily premenopausal women, physical activity at different ages did neither reduce nor increase risk of ovarian cancer. In the context of the inconsistent scientific literature, our findings probably reflect that physical activity is not causally related with ovarian cancer. © 2006 Wiley-Liss, Inc.