To develop a new decision tree system (DTS) for the management of adnexal masses in prepubertal and adolescent girls, aimed to improve the distinction between benign and malignant masses, help preserve affected ovaries during surgery, and reduce the rate of surgical management of uncomplicated functional ovarian cysts.A prospective diagnostic study using clinical and ultrasound data collected for all patients younger than 19 years of age with adnexal masses managed between 2006 and 2015.Mother and Child Health Institute of Serbia 'Dr Vukan Čupić' (Belgrade, Serbia).Patients (N = 1499) with adnexal masses, of whom 318 were surgically treated.Ultrasonographic characteristics (Ueland's Morphology Index [MI] and the ovarian crescent sign [OCS]). Patients were managed expectantly or surgically, in line with 3 rules of the DTS. Rule 1: asymptomatic patients having a mass with MI of 4 or less and OCS present, were managed expectantly. Rule 2 (emergency): malignancy was suspected if the MI was 7 or more and no edema of the OCS was present. Rule 3 (nonemergency): malignancy was suspected if the OCS was absent and MI was 5 or more. The diagnostic accuracy was assessed using sensitivity and specificity (P < .05).No malignancy was found in the group of 1236 patients selected according to the DTS rule 1. Torsion was confirmed in 36% of surgically treated masses (n = 115). The OCS was present in 96% of benign masses in the non-emergency group (n = 149) and in three with microscopic malignancy. In predicting malignancy, the DTS (rules 2 and 3) showed a sensitivity of 93 (95% confidence interval [C.I.]: 82-98); and a specificity of 97 (C.I.: 95-99). Ovarian tissue was preserved from benign (n = 254, 93%) and malignant tumors (n = 3, 7%). Only five (2%) uncomplicated ovarian cysts were surgically treated.The DTS with 3 rules is a very accurate diagnostic tool in the differentiation between benign and malignant ovarian masses. The DTS rule 1 reduces the number of surgical procedures on functional cysts, rules 2 and 3 are very useful in choosing the optimal treatment of adnexal masses, whether or not they are twisted.