We report 45 pediatric cases of Yersinia pseudotuberculosis infection confirmed by stool culture between May 1993 and June 1994. In 41 (91.1%) cases there had been contact with untreated well or mountain water. Y. pseudotuberculosis was also isolated from 4 samples of mountain spring water thought to be the sources of infection. During the course of the illness, acute renal failure (ARF) developed in 6 patients (13.6%). The age distribution of the ARF group (12.3 +/- 1.2 years) was significantly different from the non-ARF group (8.0 +/- 3.2 years). The serogroups of Y. pseudotuberculosis isolates from stool samples were 5 (n = 30) and 4 (n = 15). Isolates from the water samples were all serogroup 5. The main symptoms of both groups were fever, rash, abdominal pain, and vomiting. ARF developed between the 2nd and 14th days (mean 6 days) after the onset of fever, and oliguria (< 400 ml/m2 per day) developed in 3 patients (3/6, 50%) immediately after their fevers had subsided. ARF underwent a benign course, with complete recovery within a maximum of 4 weeks (mean 10.2 days), with 1 exceptional patient requiring hemodialysis. Renal biopsy showed evidence of tubulointerstitial nephritis. Y. pseudotuberculosis should be included as one of the causes of acute interstitial nephritis causing ARF in children, especially when the children have histories of drinking untreated water in endemic areas.