Background: Adnexal tumours are frequently diagnosed in asymptomatic postmenopausal women due to more liberal use of modern high-resolution imaging. This study's objective was to determine if there would be a difference in the intervention rates when using the Simple Rules Management Protocol (SRMP) as compared to the Risk of Malignancy Index in the Royal College of Obstetricians and Gynaecologists guideline (RMI/RCOG). Methods: This was a prospective randomised controlled trial with the participants and the researchers non-blinded, and the surgeons and pathologists blinded. We recruited pain-free postmenopausal women who were diagnosed with an adnexal tumour on ultrasound scan. Women were randomised to either of the two protocols, which then determined if they were offered conservative or surgical management. An intention-to-treat analysis was performed. The primary outcome measure was rate of surgical interventions for ovarian cysts up to 12 months after randomisation. The secondary outcome measures were the number of staging surgical procedures, surgical complications and number of delayed diagnoses of ovarian cancer. Results: A total of 148 women were randomised over 39 months with 73 in the RMI/RCOG arm and 75 in the SRMP arm with outcome data for 136 at 12 months. The two groups were balanced in terms of age, length of time since menopause and use of hormone replacement therapy. There were 18 out of 68 (28.1%) women in the RMI/RCOG arm who had surgery vs 7 out of 68 (10.3%) women in the SRMP arm (P = 0.015, chi(2)-test). The difference in these proportions was 16.2% (95% confidence interval (CI): 3.4-28.9%) and the relative risk was 2.57 (95% CI: 1.15-5.76). There were no significant differences in the number of staging surgical procedures and the surgical complications between the two groups and there were no delayed diagnoses of ovarian cancer at 12 months. Conclusions: Surgical intervention rates in asymptomatic postmenopausal women with an ultrasound diagnosis of adnexal tumours are significantly lower when the novel SRMP protocol is used for triaging compared to the standard RMI/RCOG protocol without an increase in delayed malignant diagnoses.