SUMMARY The error involved in estimating the excretion rates of white cells in urine has been shown to vary grossly. Estimations of urinary cell excretion rates may be particularly misleading when close to the limits of normality. Cell excretion rates have been correlated with the presence of pyelonephritis in eighty-three randomly selected patients. Rates below the accepted limit of 400,000 W.B.C. per hour do not exclude the presence of bacteriuria or renal damage, but a C.E.R. above this level is presumptive evidence of urinary tract pathology. Since the patient's symptoms themselves require further investigation, casual estimations of C.E.R. alone are valueless. Prednisolone by mouth in a dose of 40 mg. is effective in producing positive steroid provocation tests in patients with chronic pyelonephritis. Where other tests are inconclusive a steroid provocation test may aid in the diagnosis of chronic pyelonephritis. We are grateful to the Faculty of Medicine of the University of Edinburgh for a grant from the Gunning Medical Bursaries Fund to one of us (N. W. H. O.) to enable this work to be performed.