Abdominal actinomycosis is a rare disorder. The diagnosis is frequently missed preoperatively. We describe a patient who had had an intrauterine contraceptive device in situ for 5 years. She presented with a painful pelvicoabdominal mass, located between the uterus and rectum, and a colitis-like disease of the distal colon. Culture of the removed IUD demonstrated Actinomyces. She was intensively treated with intravenously penicillin for six weeks. Following this conservative treatment the abdominal tumor and the colitis-like symptoms totally disappeared. During two years follow-up she is free of symptoms and no signs of recurrence of the disease have been noticed. The case described here strongly underlines that Actinomyces infection should be born in mind in the differential diagnosis of a young female with an IUD, presenting with colitis, fever and an abdominal mass. Prolonged intravenously administered penicillin is warranted before surgery should be considered.