SUMMARY One hundred twenty-one standardized urine specimens from 22 patients with kidney transplants were submitted to immunoanalysis with specific antihuman fibrinogen serum, and the results were correlated with various clinical and biological events. (1) The general frequency is high (54%; normally, 0). There is a regular curve with regard to time past after transplantation. (2) In less than 25%, the precipitation lines were multiple (2 or 3), but even then immunoelectrophoretic analysis rarely showed two clear fragments. (3) No correlation was found between fibrinuria and total urinary protein output, but a significant correlation exists with a nonselective glomerular filtration of serum proteins. (4) Fibrinuria was very frequent during the acute rejection crises, especially the first 10 days of early crises, and significant correlations were found with acute kidney lesions. (5) A correlation appears to exist between a late period of fibrinuria (apart from definite crises) and late kidney lesions. (6) Significant correlations were found with fever and high dosage of prednisone. (7) In some instances noncoagulable fibrinogen-like material was found in serum in concurrence with fibrinuria. Fibrinuria is unlikely to be caused by a single mechanism. Surgical ischemia is probably contributory to early fibrinuria. Intravascular fibrinolysis has been found in some cases. Increase of glomerular permeability was noted in other circumstances. Progressive lysis of fibrin deposits in the kidney is also a probable mechanism but certainly not the only one. The phenomenon of an early fibrinuria may help to forecast the development of late lesions in kidney homotransplants.