In mid-December 2003, a cluster of 15 cases of lymphogranuloma venereum (LGV) among male homosexuals was reported to the Municipal Health Service in Rotterdam by the Erasmus Medical Centre's outpatient clinic for sexually transmitted infections (STI). Most patients presented with proctitis and some with constipation. All were Caucasian and between 26 and 48 years of age. Thirteen of them were HIV-positive and eight had a concomitant STI. All men reported having had unprotected insertive and receptive anal sexual contact. Many sexual contacts were anonymous and were reported to have taken place in Germany, Belgium, the United Kingdom and France. This outbreak of LGV may extend through a large part of western Europe. In view of the patients' international contacts, international warnings and alertness are needed. Concerted action of professionals in infectious disease control and curative care is called for.