In December 2001, the Robert Koch-Institut (RKI) was informed about a cluster of 10 Shigella sonnei infections in men who have sex with men (MSM), diagnosed in Berlin since September 2001. A retrospective investigation on sexual risk factors for infection in all shigella cases from Berlin from 2001 was initiated by sending a questionnaire to all patients without known travel history. Simultaneously laboratories were asked to send new shigella isolates from patients to the National Reference Centre at the RKI. Out of 29 responders, 24 self-identified as MSM. Seventeen of these reported sexual contacts as the most likely potential risk of infection. Almost all MSM reported direct or indirect oral-anal sexual contacts in the week before they fell ill. Fifteen of 27 shigella isolates collected during the prospective sampling period showed identical plasmid profiles, PFGE- and resistance patterns and can therefore be regarded as clonally identical. Asymptomatic and/or prolonged shedding in the reconvalescent phase may contribute to the transmission risk of shigella infection during oral-anal sexual practices. Awareness among practitioners and among MSM about the risk of sexual transmission of orally transmitted agents needs to be raised.