A group of 50 patients with secondary forms of pyelonephritis was treated with nitrofurantoin (Furadantin^R, Furadantin^R-retard). In all these patients, serious anatomical alterations and functional disorders had been corrected by operative procedures 6 months before onset of this treatment. After an initial high-dose treatment and subsequent low maintenance doses for some months, the secondary bacterial pyelonephritis was cured and relapses were prevented. The criteria adopted for the clinical cure of secondary pyelonephritis comprised the repeated finding of a sterile urine and the disappearance of the pyuria present before treatment, as well as normalization of the creatinine values and the ESR. Patients with creatinine values above 1.5 mg/100 ml were not included in this investigation. Since long-term treatment necessitates the use of a safe drug with the least possible subjective side effects, Furadantin-retard is highly suitable for this purpose.