There are only sparse data on the short-term and medium-term clinical impacts of Campylobacter concisus infection. A clinical study was performed during a 2-year period to determine the clinical manifestations in C. concisus-positive adult patients. A case patient was defined as an adult patient (≥18 years) with a C. concisus-positive stool sample during the study period. Clinical data were obtained with use of a questionnaire supplemented with the patients' medical records, if any. The short-term and medium-term clinical manifestations in these patients were compared with those of patients with Campylobacter jejuni/Campylobacter coli infection. One hundred and seventy-four C. concisus patients and 196 C. jejuni/C. coli patients participated in the study. Patients with pre-existing inflammatory bowel disease or microscopic colitis or enteric co-infection were excluded from review of the clinical manifestations. Comparison of the short-term clinical manifestations in 139 C. concisus patients with those in 187 C. jejuni/C. coli patients showed a significantly lower prevalence of fever, chills, mucus and blood in stools, and weight loss. However, 80% of C. concisus patients, but only 32% of C. jejuni/C. coli patients, had diarrhoea for >2 weeks. After a 6-month follow-up period, 12% of C. concisus patients were diagnosed with microscopic colitis, whereas no C. jejuni/C. coli patients were diagnosed with non-infective colitis. Irritable bowel symptoms were common in both groups at follow-up. C. concisus infection seems to cause a milder course of acute gastroenteritis than C. jejuni/C. coli infection, but is associated with more prolonged diarrhoea.