A long-term follow-up analysis of the randomized clinical trial Adjuvant Chemotherapy in Ovarian Neoplasm (ACTION) from the European Organization for Research and Treatment of Cancer was undertaken to determine whether the original results with a median follow-up of 5.5 years could be verified after longer follow-up with more events. In the ACTION trial, 448 patients with early ovarian cancer were randomly assigned, after surgery, to adjuvant chemotherapy or to observation (no further treatment). The original analysis found that adjuvant chemotherapy improved recurrence-free survival but not overall survival and found in a subgroup analysis that completeness of surgical staging was an independent prognostic factor, with better recurrence-free and overall survival among those with complete (optimal) surgical staging. After a median follow-up of 10.1 years, we analyzed the more mature data from the ACTION trial and found support for most of the main conclusions of the original analysis, except that overall survival after optimal surgical staging was improved, even among patients who received adjuvant chemotherapy (hazard ratio of death = 1.89, 95% confidence interval = 0.99 to 3.60; overall two-sided log-rank test P = .05). More cancer-specific deaths were observed among nonoptimally staged patients (40 [27%] of the 147 deaths in the observation arm and 11 [14%] of the 76 deaths in the adjuvant chemotherapy arm) than among optimally staged patients (seven [9%] of the 75 deaths in the observation arm and 11 [14%] of the 76 deaths in the adjuvant chemotherapy arm) (two-sided chi(2) test for heterogeneity, P = .06). Thus, completeness of surgical staging in patients with early ovarian cancer was found to be statistically significantly associated with better outcomes, and the benefit from adjuvant chemotherapy appeared to be restricted to patients with nonoptimal surgical staging.