A case-control study was done to determine the risk and pathogenesis of febrile spontaneous abortion for intrauterine device (IUD) wearers compared to non-wearers. Four groups of women coming to a large city hospital between 1970 and 1975 were compared: 1) women presenting with febrile spontaneous abortion, 2) women presenting with afebrile spontaneous abortion, 3) women presenting with afebrile spontaneous abortion which subsequently became febrile, and 4) women delivering a live infant. Pregnant women with an IUD in situ had a 5-fold higher risk of both febrile and afebrile spontaneous abortion compared to pregnant women without an IUD in situ. Women who had a spontaneous abortion that shifted from afebrile to febrile were somewhat more likely to be IUD wearers than the other 2 spontaneous abortion groups. In our study population, the increased risk of febrile spontaneous abortion for IUD wearers appeared primarily due to the increased risk of the spontaneous abortion event itself, rather than a primary IUD-related infection causing febrile spontaneous abortion.