Ultrasound in the investigation of the right iliac fossa mass. Patients presenting with a right iliac fossa (RIF) mass are a diagnostic problem. The objective of this study was to assess the role of ultrasound (US) in their investigation. A prospective series of 50 patients presenting with a clinically suspected RIF mass was examined by US and the finding correlated with the final diagnosis. There was a positive finding in 34 patients (68%). Ultrasound correctly identified the organ of origin in 33 (97%) and was able to guide the patients' further management. In 12 cases no abnormality was found in the RIF, of which 11 had no positive findings at follow-up and one was shown to have an unrelated abnormality at laparotomy. In four cases the findings were due to normal variants. Ultrasound is the imaging modality of first choice in patients presenting with a RIF mass. 