Despite the introduction of new intrauterine devices (IUDs), the most important complication involving their use continues to be excessive menstrual bleeding. IUD use in developing countries with women who are already depleted in body iron stores may prove to be deleterious to their health. Mean amounts of menstrual blood loss (MBL) for women not using contraception in the western world is about 32 ml. This mean is increased to 52-72 ml with use of the Lippes loop and other non-medicated devices up to 24 months after insertion. For the Copper-7 and Copper-T-200 devices this mean increase is to 37-40 ml in the first month, decreasing to 30-38 ml at 12 months after IUD insertion. In the users of the Multiload-250 IUD at one month post-insertion the MBL is from 56 to 63 ml and from 36 to 39 ml at 24 months of use. The mean Multiload-375 device users at one month after insertion lose a mean of 45-73 ml at 24 months, 35-50 ml. With the progestogen-releasing IUD mean MBL is 27-36 ml at 1 month and 9-13 ml at 12 months post-insertion. Intermenstrual blood loss is significant only in the first month of use for all IUDs. Discontinuation rates for pain and bleeding with non-medicated IUDs are from 11.0-19.6 per 100 women per year, and for the copper IUDs 4.4 to 6.8 per 100 women in the first year of use. The main problem with prolonged menstrual bleeding is depletion of the body iron stores; this is highly significant with non-medicated devices, less important with copper devices and conversely, iron stores are increased in users of progestogen-releasing devices. This is based on serum ferritin measured up to 24 months after insertion. The ferritin values correlated well with the volumes of MBL. It is suggested, especially for women with low body iron stores, that there is an order of preference for IUDs to be used. This should be: firstly, the progestogen-releasing devices; secondly, the Copper-T and Copper-7 IUDs; thirdly the larger surface copper devices (Cu-T-220C, Multiload 250 and 375, Cu-T-380). Non-medicated devices are not to be recommended for these women.