Investigation of kidney-biopsy taken from 17 children was carried out. The techniques used were: light-microscopic investigation of paraffin sections and ultrathin sections, electron microscopic investigation of the latter and basal-membrane measurements. According to the author's opinion electron microscopic investigation of kidney-biopsy is reasonable only in selected cases, representing special clinical and therapeutic problems. In cases of nephrosis syndrome electron microscopy helps in differentiation of the "mimimal lesion" and the early phase of the focal sclerotizing glomerular lesion. Further it gives good results in observation of mono- and olygosystemic diseases of the kidney. In the author's material electron microscopy contributed to more differentiated morphological diagnosis in 2/3 rds of the cases investigated. The authors join to the opinion of other investigators stressing that light-microscopical investigation should be completed by the light-microscopy of uktrathin scretions even then, when there are no possibilities of electonmicroscopical study.