The discovery of a complex adnexal mass in an older woman often raises concern for cancer. We evaluate outcomes for a large population-based cohort of women older than age 50 years with a small complex adnexal mass reported on ultrasound, without elevated CA125 or other evidence of malignancy, including time to detection of malignancy and stage at diagnosis for those initially observed. Women older than age 50 years who had an ultrasound during 2007-2011 reporting a complex adnexal mass 1-6 cm in size were identified. Previous or subsequent pelvic ultrasounds were reviewed to determine when the mass was first identified and whether there was change over time. Women with concurrent elevated CA125, evidence of metastatic disease, or less than 24 months of clinical follow-up were excluded. Surgical pathology from removal and diagnoses of ovarian cancer within 24 months of follow-up were identified. Among 1363 complex masses identified, 18 cancers or borderline tumors (1.3%; 95% confidence interval, 0.8-2.1%) were found. Six cases were diagnosed among 204 women who had immediate surgery after initial ultrasound (15%), and 12 additional cases were found among 994 women with at least 1 repeat ultrasound (73%). Growth was apparent on ultrasound by 7 months for all borderline and epithelial ovarian cancers. Of the 12 cases diagnosed during follow-up, 10 were found to be stage 1 at surgery. Among isolated adnexal masses reported as complex and 1-6 cm on pelvic ultrasound in women older than 50 years, the overall risk of malignancy is low. All cases of epithelial cancer and borderline tumor demonstrated growth by 7 months of observation.