PURPOSE: Multiple trials by the World Health Organization have established levonorgestrel as the gold standard in hormonal emergency contraception (EC). However, changes in menstrual patterns following EC have been observed; thus, we undertook this prospective study to identify and determine the characteristics of these changes. MATERIALS: Women requesting EC at either any of two hospitals --1 family planning unit and 12 pharmacies in Yaound?Cameroon were enrolled if they had a history of regular menstrual cycles over the previous 3 months and if they agreed to follow-up until the end of the subsequent menstrual cycle. Pretreatment menstrual patterns were compared with those of the EC treatment cycle and the cycle after EC. RESULTS: In a set of 232 participants (mean age, 25 years), we observed 34 (14.7%) cases of incident intermenstrual bleeding and statistically significant changes in menstrual cycle length, menstrual period length and menstrual appearance compared to baseline patterns that differed according to whether EC was taken well before, close to or well after the expected ovulation for that cycle. The majority of these changes disappeared in the following cycle. CONCLUSION: Levonorgestrel EC is associated with significant but transient changes in menstrual patterns in a significant proportion of users.