The general policy of making a complete urologic examination of children suffering with chronic pyuria has brought to light an interesting and as yet poorly classified group of pathologic conditions. In the majority of instances, an obstruction at some portion of the primary tract The Direct Pathologic Effects of Obstruction has been the primary factor in the chronicity of the pyuria. The obstructive syndrome is probably the same in children as in adults. Although pain is not so prominent, the sequence of changes of obstruction-paininfection obtains, and these effects are due to various causes of back pressure, stasis or back-flow, just as in adults. Infection is always imminent in the presence of any of these, and the clinical effects in their relation to the pathologic changes are diagrammatically represented in the foregoing outline. The nature of the causes that are known to produce these local, general and functional effects is