PURPOSE: To evaulate the volume change of peritoneal inclusion cysts on the follow-up ultrasonography(US). MATERIALS AND METHODS: From March 1995 to May 1999, thirty seven women with ultrasonographically diagnosed peritoneal inclusion cysts were included in this study. Six patients underwent surgery for several reasons. Follow-up ultrasonography was performed 70-456 days (mean=191 days) after initial US examination in 12 of the remaining 31 patient with no further treatment. US was performed with a 3.5 or 4 MHz transabdominal probe in all 18 patients who underwent either surgery or follow-up US, and additional tranvaginal US examination using a 5-7 MHz probe in 15 of 18 patients. The volume change of the cysts was recorded for each US examination. RESULTS: Three cysts (25%) (volume=170 cm3, 61 cm3, and 38 cm3, respectively) were completely resolved on the follow-up US while the other nine cysts showed a decresed volume in seven patients (58%) and increased volume in two patients (17%). CONCLUSION: Spontaneous regression of peritoneal inclusion cysts is more common than it is believed to be, and ultrasonography may be a useful follow-up examination for peritoneal inclusion cysts.