Purpose. The aim of this study was to evaluate kidney length in patients with postoperative acute renal failure (PARF). Methods. The effect of PARF on renal size was prospectively studied in 76 patients with PARF and 40 healthy volunteers. Sonographic measurements of kidney length and the level of serum creatinine were obtained each day patients stayed in our surgical intensive care unit. These measurements were done once in volunteers. Ail study subjects were divided into groups on the basis of age, those younger than 65 years and those 65 years or older. Statistical analyses on the relation of renal size, age, and degree of PARF used the kidney length:body height ratio (KBR) and the peak serum creatinine level. Follow-up kidney length and creatinine measurements were done in 24 patients 1-5 years after they recovered from PARF. Results. Regardless of age, mean KBRs were significantly greater in patients than in healthy volunteers (< 65 years, p < 0.001; greater than or equal to 65 years, p = 0.008), with a negative correlation between KBR and patient age (r = -0.664; p < 0.001). A positive correlation was found between the KBR and the peak serum creatinine level in patients younger than 65 years (r = 0.543; p < 0.001); an insignificant negative correlation was found between these factors in patients 65 years or older (r = -0.264; p = 0.1). Follow-up on recovered patients showed that their KBRs were significantly lower than the values when patients had PARF (< 65 years, p < 0.001; greater than or equal to 65 years, p = 0.027). Conclusions. PARF produces a sonographically measurable increase in renal size.