Context: Screening for ovarian cancer with cancer antigen 125 (CA-125) and transvaginal ultrasound has an unknown effect on mortality. Objective: To evaluate the effect of screening for ovarian cancer on mortality in the Prostate, Lung, Colorectal and Ovarian (PLCO) Cancer Screening Trial. Design, Setting, and Participants: Randomized controlled trial of 78 216 women aged 55 to 74 years assigned to undergo either annual screening (n=39 105) or usual care (n=39 111) at 10 screening centers across the United States between November 1993 and July 2001. Intervention: The intervention group was offered annual screening with CA-125 for 6 years and transvaginal ultrasound for 4 years. Participants and their health care practitioners received the screening test results and managed evaluation of abnormal results. The usual care group was not offered annual screening with CA-125 for 6 years or transvaginal ultrasound but received their usual medical care. Participants were followed up for a maximum of 13 years (median [range], 12.4 years [10.9-13.0 years]) for cancer diagnoses and death until February 28, 2010. Main Outcome Measures: Mortality from ovarian cancer, including primary peritoneal and fallopian tube cancers. Secondary outcomes included ovarian cancer incidence and complications associated with screening examinations and diagnostic procedures. Results: Ovarian cancer was diagnosed in 212 women (5.7 per 10 000 person-years) in the intervention group and 176 (4.7 per 10 000 person-years) in the usual care group (rate ratio [RR], 1.21; 95% confidence interval [CI], 0.99-1.48). There were 118 deaths caused by ovarian cancer (3.1 per 10 000 person-years) in the intervention group and 100 deaths (2.6 per 10 000 person-years) in the usual care group (mortality RR, 1.18; 95% CI, 0.82-1.71). Of 3285 women with false-positive results, 1080 underwent surgical follow-up; of whom, 163 women experienced at least 1 serious complication (15%). There were 2924 deaths due to other causes (excluding ovarian, colorectal, and lung cancer) (76.6 per 10 000 person-years) in the intervention group and 2914 deaths (76.2 per 10 000 person-years) in the usual care group (RR, 1.01; 95% CI, 0.96-1.06). Conclusions: Among women in the general US population, simultaneous screening with CA-125 and transvaginal ultrasound compared with usual care did not reduce ovarian cancer mortality. Diagnostic evaluation following a false-positive screening test result was associated with complications. Trial Registration: clinicaltrials.gov Identifier: NCT00002540. ©2011 American Medical Association. All rights reserved.