A waterborne Campylobacter jejuni outbreak in the Rheumatism Foundation Hospital in Heinola, Finland, in November-December 1986 is described. 32 patients and 62 members of the staff developed gastrointestinal symptoms. C. jejuni heat-stable serotype 45 was isolated from the faeces of 32 enteritis patients and from none of the controls. No other enteropathogens were found. Positive serological responses to C. jejuni acid extract antigen were detected by enzyme immunoassay in 34% of the symptomatic hospital patients, in 40% of the symptomatic staff members, and in 10% of the controls. The clinical course of the illness was mostly mild and self-limited. No striking progress in the arthritis symptoms of the patients was found after the outbreak. The hospital has its own water supply. C. jejuni of the same serotype as the epidemic strain was isolated from the water of the pipeline system. After a careful examination some aged components of the waterworks were found to be responsible for leaks that resulted in the contamination of the water.