Level diagnosis repeatedly performed in patients without roentgenologically demonstrable reflux demonstrated bladder bacteriuria in 80% of the cases. The remaining 20% had supravesical bacteriuria. We called this occult reflux, if reinfection was demonstrated. Contamination of the upper tract by occult reflux can, but must not induce pyelonephritis. Bilateral antireflux surgery frequently eliminates occult reflux of bacteria, so this seems a debatable method of treatment.