In 26 cases of myelofibrosis, the authors investigated for possible renal impairment that can be appraised from the usual clinical, laboratory, and roentgenographic signs. No anomalies were demonstrated in 12 of these cases. In 14 (or 53%) of the patients, some anomaly was discovered : essentially proteinuria with minor alteration of renal function, but also, two cases of poorly functioning left kidney evidenced on intravenous urograms, one case of acute anuric renal failure connected with hyperuricemia, one case of hypokalemic tubulo-interstitial nephritis, and one case of glomerulonephritis with, nephrotic syndrome. This study, when compared to the literature, indicates that besides nephropathy specific to myelofibrosis and attributed to myeloid metaplasia in the kidney, serious consideration must be given to lesions due to (1) compression of the left kidney by the enlarged spleen, (2) urate precipitation in the urinary passages, and (3) a possible glomerular disorder whose mechanism remains undefined.