The IUD ML Cu375 was inserted, after bacteriological screening, into 620 women who were observed for 12 months. Bacterioscopy and, when needed, bacteriology of vaginal smears were performed 7 days, and 1, 3, 6 and 12 months after insertion. During the 12 months, pelvic inflammatory disease (PID) was diagnosed in 4 patients (0.6%) and sexually transmitted disease (STD) in 73 patients (11.8%). Careful selection of patients and bacteriological screening can effectively reduce the risk of bacterial contamination and subsequent development of PID.