PIP: The conversion of sales statistics for IUDs into number of users requires assumptions about their average duration of use, which may have changed over the past few years. 308 responses to a mail questionnaire sent to physicians regularly prescribing IUDs were analyzed to obtain an up-to-date estimate of average duration of use. The response rate of 30% was reasonable for a mail questionnaire. The geographic and sex distribution of respondents were on the whole very similar to those of all obstetrician-gynecologists in France, but there were fewer than expected respondents under age 40. The respondents inserted an average of 20 IUDs per month. In 1978, over 3/4 of physicians who responded to a National Institute of Demographic Research survey reported removing IUDs after 24 months, and the average duration of use of an IUD was 26-27 months. By 1985, only 1/3 of physicians planned to remove IUDs after only 24 months. The average duration of use increased to 33-34 months. Female doctors and those under 45 were particularly likely to remove IUDs after longer periods of use. Different formulas for estimating the number of IUD users were compared. The 1st formula, which requires sales data for 3 consecutive years and assumes an average duration of use of 1.9 years, with no variation over the 3 years, yielded estimates of 120,000 users in 1971 and 912,000 in 1978, values in agreement with survey estimates for those years. The number of users for 1985 according to the formula would be about 1.8 million. Another formula allowing for fluctuation of sales figures over 5 years and a progressive lengthening of the duration of use of each device assumed an average duration of use of about 1.95 years. This formula and a simplified version yielded estimates of the number of users in 1985 of about 2.2 million, or 16-17% of all French women aged 15-49 years. Still another formula, which differentiates between users who have their IUDs removed almost immediately, users leaving their devices in place as long as possible, and users who are regularly followed, yields an estimate of 1.95 million users + or - 10%. The number of new users of IUDs in France appears to have stabilized after 1981. About 13-15% of fertile aged women are estimated to use IUDs.