There is one renal disease in which unilateral estimation of GFR is desirable for clinical purposes: chronic pyelonephritis. Considering the possibilities for complications to determine unilateral renal function by ureter catheter just in this group of patients, we tried to estimate unilateral GFR with 66mTc-DTPA after a single injection of 5 mCi i.v. on the basis of blood samples at 90, 110 and 130 min after injection and evaluation of gamma-camera data from 25 to 35 min after administration of the dose. 31 patients were studied. In arenal patients clearance values were 7.5 ml/min (+/- 10.47 ml/min), unilaterally nephrectomized patients showed zero values for the removed kidney. Normal patients had a mean GFR of 107 +/- 17.7 ml/min with a partition between right and left kidney of 54.2 +/- 10 to 52.75 +/- 8.5 ml/min. Patients without urinary tract obstruction and unilateral renal disease had values of 27.1 +/- 11 ml/min for the diseased kidney while the normal kidneys gave a mean value of 59.6 ml/min (+/- 13 ml/min). In the presence of urinary tract obstruction, data could not be correlated to the state of renal function. This shows that unilateral estimation of GFR using 99mTc-DTPA is basically possible, but that it is useless in obstructive kidney disease.