Ninety-four women attending a family planning clinic for emergency contraception (EC) were asked how certain they were of the date of their last menstrual period (LMP), of the timing of intercourse, and how many times in the cycle they had had sex. Urinary pregnanediol concentrations were analyzed in 64 women to assess whether they had ovulated before they used EC. Forty-five women were certain of the date of the LMP, the rest were not. Only four women could not accurately recall the timing of intercourse, and 60% had had intercourse more than once in the cycle. Twenty-one women had urinary pregnanediol concentrations that were inconsistent with their cycle day. Calculations of the efficacy of EC depend on knowing the timing of intercourse in relation to the estimated day of ovulation. The results of this study suggest that these calculations are likely to be inaccurate for a significant minority of women.