For 3 years a filtration system for the isolation of "new" campylobacter was included in the culture protocol of 15,185 stool specimens. "C upsaliensis" was isolated in 99 patients, C jejuni subsp doylei in 4, and C hyointestinalis in 2. "C upsaliensis" was the only organism isolated in 83 patients. Clinical information was available for 77 out of these 83 patients. 92% of the patients had diarrhoea; vomiting and fever were rare (14% and 7%, respectively); the onset was mostly sudden; and the symptoms usually lasted for less than a week. Gross or occult blood was present in a quarter of cases and neutrophils were detected in faecal smears in about a fifth. "C upsaliensis" may be an unrecognised and frequent cause of diarrhoea in man, and selective isolation media should be combined with non-selective isolation systems.