Objectives To investigate outcomes of patients with complex cystic breast lesions and to determine malignancy rates for each sonographic category. Material and Methods A total of 124 women with a mean age of 45 were enrolled in this retrospective study. Complex cysts were classified into four categories according to sonographic findings (thick septa/wall, intramural nodule, solid/cystic ratio) by two radiologists. Size, margin, short‐to‐long axis ratio, and vascularity were also noted. The relationship between sonographic features and histopathological/follow‐up results was evaluated. Results Of 124 complex cysts, 11 were malignant and 113 were benign. There were 31 with thick septa/wall (category 1), 14 with intramural nodule (category 2), 60 with solid/cystic ratio < 1 (category 3), and 19 with solid/cystic ratio > 1 (category 4). Among 11 malignant tumors, six intraductal papillary carcinomas and three invasive ductal carcinomas were in category 3, whereas one invasive ductal carcinoma was in category 1 and one was in category 4. Most of malignant lesions (9/11, 82%) were in category 3. Most of benign lesions (96/113, 85%) were fibroadenomatoid/fibrocystic changes. The presence of vascularity (96%), irregular margin (94%), and short‐to‐long axis ratio ≥ 1 (94%) had higher accuracy rates for diagnosis of malignancy. Conclusion The malignancy rate was 8.8% in our study. Besides internal morphological features, other sonographic features should be considered in the evaluation of complex cystic breast lesions. Due to substantial risk of malignancy and the overlapping sonographic features of benign and malignant lesions, tissue sampling of all complex cysts should be performed.