OBJECTIVE The aim of this study was to assess the natural history of ultrasonically diagnosed ovarian dermoid cysts in a large group of women who were managed expectantly, and to assess the factors that were associated with failure of expectant management. METHODS Our database was searched for dermoid cysts diagnosed on ultrasonography by a single expert operator between 2001 and 2007 in this retrospective study. In women who opted for expectant management, demographic data including age, gravidity and parity were recorded. Indications for scan, site of cysts, dimensions and the outcomes of expectant management were also recorded. RESULTS Two hundred and eighty-nine women were diagnosed with a total of 323 dermoid cysts by a single expert ultrasound operator. 93/289 (32.2%; 95% CI, 26.8-37.6%) women with 105/323 (32.5%; 95% CI, 27.4-37.6%) ovarian dermoid cysts were managed expectantly for longer than 3 months. The mean age at diagnosis was 33.8 (range, 13-79) years and the median duration of follow up was 12.6 (interquartile range, 7.6-29.3) months. The mean growth rate of dermoid cysts during follow up was 1.67 mm/year. There were no demographic or morphological features that could be used to predict the growth rate of dermoid cysts. After a period of expectant management, 24/93 (25.8%; 95% CI, 16.9-34.7%) women had surgical intervention. The risk of surgical intervention was significantly increased in younger women, those of parity > or = 2 and in women with bilateral cysts or larger-diameter cysts, and reduced in women with a past history of ovarian cyst. CONCLUSIONS The success rate of expectant management of dermoid cysts is high and this approach should be considered as a viable alternative to surgical management.