The efficacy of administering an antibiotic prior to IUD insertion to reduce the risk of introducing an upper genital tract infection during the procedure has not yet been established. Two double-blind randomized studies conducted in Africa comparing a 200 mg prophylactic dose of doxycycline with a placebo did not conclusively identify a reduced risk of post-insertion pelvic inflammatory disease (PID). A clinical trial of comparable design is currently under way in the US. This multi-site trial will evaluate whether use of an antibiotic prior to insertion reduces the risk of IUD removal for all medical reasons, including upper genital tract infection, within the first three months after insertion. This paper reports on the pilot phase of this study, which was designed to test the protocols and data collection instruments in advance of the full-scale clinical trial. A total of 447 prospective IUD (TCu-380A) users were randomly assigned to receive either a 200 mg dose of doxycycline or a placebo one hour before IUD insertion. 3.6% (8/219) of participants who received the antibiotic had the device removed for medical reasons (infection, bleeding, cramping, etc.) within three months post-insertion compared to 4.5% (10/223) of participants who received the placebo. This reduction in the removal rate was not statistically significant given the limited size of the pilot study (RR = 0.81; 95% CI 0.28-2.29). Only two subjects, one from each treatment group, met the diagnostic criteria for acute PID. The overall three-month retention rate was 91.8% for the antibiotic group and 89.7% for the placebo group.(ABSTRACT TRUNCATED AT 250 WORDS)