OBJECTIVE To evaluate the clinical significance of sonographically detected fluid collections following Cesarean section and hysterectomy, and to identify risk factors associated with their formation. METHODS This was a prospective study including 280 women, 145 of whom had undergone a Cesarean section and 135 of whom had undergone abdominal or vaginal hysterectomy. Ultrasound examinations were carried out on all women on day 4 after surgery to assess the presence of abdominal wall or pelvic fluid collections. The sonographers were unaware of the clinical course before the examination and were not involved in any clinical decision-making. Ultrasound findings were correlated with clinical data and postoperative morbidity. RESULTS A fluid collection was found in 69 (48%) women after Cesarean section, and in 59 (44%) women who had undergone hysterectomy. No risk factors for the development of fluid collections after Cesarean section or hysterectomy were identified. The risk of developing febrile morbidity was not related to the presence, location or size of fluid collections. CONCLUSIONS Postoperative fluid collections are common after Cesarean section and hysterectomy. As fluid collections detected by sonography were not associated with postoperative morbidity, this finding is unlikely to be useful in the workup for postoperative fever.