An outbreak of diarrhoea due to Salmonella virchow phage type 8 occurred in a major block of a large psychiatric hospital. The two other major blocks of the hospital remained unaffected. No source of infection was identified and the epidemiological investigations pointed to cross-infection as the mode of transmission. Infection control measures were implemented at an early stage but a total of 55 patients and four members of staff were affected. The disease was self-limiting and of short duration. During the course of the outbreak, Salmonella typhimurium phage type 66 also emerged as a pathogen and was isolated from the stools of symptomatic cases with diarrhoea. Owing to the size of the hospital, the lack of facilities for infection control, the nature of the patients' illnesses and an increasing number of affected patients, it was decided to treat all symptomatic and asymptomatic patients with oral ciprofloxacin 500 mg twice daily for 7 days. A quantitative stool culture for salmonellae was undertaken on days 3, 7, 10, 30, 60 and 90 from the start of treatment. All patients' and staff's stool cultures became negative on day 7 and remained negative for a period of 6 months during follow-up. We conclude that cross-infection outbreaks due to Salmonella spp. in psychiatric and geriatric hospitals can be controlled by isolation in a designated ward and supportive therapy, together with ciprofloxacin treatment.