Background: The objective was to assess whether expectant management of sonographically benign ovarian cysts may be an option for selected asymptomatic premenopausal women. Methods: This is a prospective observational longitudinal study. Between January 1997 and December 2002, 323 asymptomatic premenopausal women (mean age: 40.6 years; range: 19-50 years) diagnosed as having a sonographically benign ovarian cyst measuring <6 cm were offered conservative management with periodic follow-up at 6-12 month intervals. In all cases, a first check was performed 3 months after diagnosis to confirm the 'persistent' nature of the cyst. A total of 120 women agreed to participate in this study and constitute the basis of the data presented. Results: Mean diameter at diagnosis for the most frequent lesions were as follows: endometrioma 3.3 cm (SD 1.5); simple cyst 4.1 cm (SD 1.6); dermoid cyst 3.2 cm (SD 1.4); haemorrhagic cyst 3.5 cm (SD 1.2); hydrosalpinx 2.9 cm (SD 1.0). With a median follow-up of 42 months (range: 18-94 months), most lesions remained unchanged, both in size and sonographic appearance. Ten cysts (8.3%) disappeared during follow-up, all of them after more than 2 years of follow-up. No patient has developed signs or symptoms suggesting ovarian cancer. Conclusion: Most sonographically benign ovarian cysts remain unchanged during long-term follow-up. Our data would support conservative management in these cases. © The Author 2005. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved.