A female patient of child-bearing age using an intrauterine device (IUD) consulted our department for a hypogastric mass which turned out to be actinomycosis of the bladder. Antibiotic treatment resolved the infection and removal of the inflammatory mass was unwarranted since infected fistulous tracts had been observed. There appears to be a causal relationship with IUDs, as in genital actinomycosis. Due to the increasing use of contraceptive devices, we are likely to find this type of infection, which might be difficult to distinguish from malignant disease, increasingly frequent in urologic practice.