A prospective study was carried out to determine the value of daily C-reactive protein (CRP) measurements in the diagnosis of acute rejections and other complications in the immediate post renal transplant period. Sixty-nine renal allograft recipients were studied. The CRP level was considered to have increased during an acute rejection episode if it rose above baseline values (greater than 10 micrograms/ml) within +/- 3 days of the initiation of antirejection therapy. Sixteen of 33 patients treated with azathioprine and prednisone had at least 1 acute rejection. Twelve of the 16 displayed a significant rise in CRP (mean increase 40 micrograms/ml) but in only 8 did the rise occur prior to or on the day of rejection diagnosis. Seventeen of 36 patients treated with Cyclosporine A and prednisone had at least 1 acute rejection; only 8 of these patients experienced a significant rise in the CRP (mean increase 27 micrograms/ml) and in only 5 of these did the increase occur prior to or on the day of rejection diagnosis. Serious complications other than acute rejection occurred in 24/69 patients. The mean peak CRP value was 114 micrograms/ml during these complications and was significantly higher (p less than 0.001) than the peak CRP found in the rejection group (35 micrograms/ml). Furthermore, the CRP was equal to or greater than 60 micrograms/ml during 23/24 non-rejection complications but only reached this value during 4 of the 33 acute rejections.(ABSTRACT TRUNCATED AT 250 WORDS)