Fibronectin (FN) levels were determined in 64 cases with chronic renal failure (CRF), some of whom were undergoing dialysis. FN levels were 14.9 +/- 7.6 mg/dl in CRF (n = 20), 13.4 +/- 4.3 mg/dl in patients on continuous ambulatory peritoneal dialysis (CAPD) (n = 20) and 16.7 +/- 7.2 mg/dl in patients on hemodialysis (HD) (n = 24). All the levels were significantly lower than in normal subjects (23.1 +/- 4.6 mg/dl). Serum FN was compared with some nutritional indices. Positive correlations were found between serum FN and nitrogen balance (BN), serum prealbumin (PreA) and transferrin (Tf) in all the patients. With serum albumin (Alb), however, this correlation was only found in patients undergoing dialysis. Negative correlations were found between serum FN and the ratio of serum urea to serum creatinine (Surea/Scr) in CAPD and HD patients. In 10 CAPD patients, the low serum FN levels went up after increased protein intake. This indicates that it was the result of malnutrition due to decreased protein intake. Serum FN level reflects a negative BN earlier and better than serum PreA, Tf and Alb. It is a sensitive, reliable and simple index for judging the nutritional protein status and the effect of nutritional treatment in patients with CRF undergoing dialysis.