A prospective study was undertaken to evaluate the incidence, course, effects of treatment and outcome of patients with Yersinia enterocolitica infections. A total of 189 patients were followed: 62.5% had enteric forms of illness, 20.6% extramesenteric forms, 23.2% arthritis and erythema nodosum. Lymphadenopathy with high fever and weight loss, a septic syndrome and hepatitis were predominant manifestations of the extramesenteric form. Ten per cent of the isolates (135) were susceptible to amoxycillin (4 mg/l), 38% to cephradine (8 mg/l), 82% to doxycycline (4 mg/l), 83% to chloramphenicol (4 mg/l), 85% to trimethoprim (1 mg/l), 87% to cefuroxime (8 mg/l), 92% to piperacillin (16 mg/l), 99% to gentamicin (1 mg/l) and 100% to cefotaxime (4 mg/l), pefloxacin (0.12 mg/l), ofloxacin (0.06 mg/l) and ciprofloxacin (0.016 mg/l). The majority of the patients with enteritis recovered without antibiotic therapy. The duration of enteritis was not significantly influenced by antibiotic treatment. Eighty five patients, 46 with enteric and 39 with extramesenteric forms were treated. The clinical response to co-trimoxazole was 71%, and to doxycycline 75%. Cefuroxime, ceftazidime, cefoperazone, piperacillin and gentamicin failed in seven of eight courses. Three patients treated with ciprofloxacin responded well. The role of quinolones in yersiniosis needs further attention.