We analysed the results of clinical and morphological examinations in 256 children with mesangial glomerulonephritis. In the majority of children the onset of the disease was associated with the nephrotic syndrome, less frequently with erythrocyturia, proteinuria plus erythrocyturia or proteinuria. The course of the disease was markedly worse in those with proteinuria. The patients in this group were most frequently submitted for repeated biopsy. Renal specimens were studied in light, electron and immunofluorescence microscopy. Certain relationships between the initial symptoms and composition of deposits were found. There was, however no relation between the presence or absence of the deposits or their composition and the course of the disease. In 30 children the results of first biopsy (mainly electron microscopy examinations) suggested a possibility of focal segmental glomerulosclerosis (FSG). In 11 out of these children biopsy was repeated confirming the results of the first one. Repeated biopsy was performed in a total of 22 children. Progression of changes (signs of FSG) was found in 19 children, in 2 children changes in repeated biopsy did not differ from those in first biopsy, and in one child regression was observed.