Three children were diagnosed as having acute renal failure due to acute tubulointerstitial nephritis (TIN) associated with Yersinia pseudotuberculosis infection. The kidneys were observed ultrasonically during the disease course. The subjects presented with remittent fever, vomiting or diarrhea, and nonoliguric acute renal failure with sterile pyuria and tubular reabsorptive dysfunction. Ultrasound examination delineated marked enlargement of the kidneys with diffuse hyperechogenicity in the cortex, suggesting the diagnosis of TIN. The renal swelling was marked in the descending order of width, depth and length, respectively, and reached more than 200% in volume in 1 of the children who was examined at the culminating stage of acute renal failure. The enlarged kidneys reduced in size in parallel as the elevated serum creatinine levels decreased. However, the enlarged kidneys did not seem to return to their original sizes: the kidney size in 1 of the children remained well above the upper allowance limit of the body-height-related normals and in the other 2 they were only a little smaller than the upper limits. These ultrasound findings should help with the diagnosis and follow-up of the disease.