Rapidly progressive glomerulonephritis is a disease characterised by an abrupt drop in glomerular filtration rate in a short period of time, which is caused by crescent formation seen in at least 50% of glomeruli. Two cases presented below illustrate rapid progressive glomerulonephritis in adolescents as a disease of heterogeneous aetiology that can develop both in the course of primary glomerulopathies and glomerulopathies secondary to systemic vasculitis. In the first case of an 11-year-old girl, nephritic syndrome with renal failure was accompanied by the presence of anti-myeloperoxidase antibodies in the serum, which in combination with the histopathological picture of the kidneys indicating pauci-immune rapidly progressive glomerulonephritis was the basis for the diagnosis of renal limited vasculitis. In the second case of a 16-year-old boy, an adverse course of acute post-streptococcal glomerulonephritis with features of severe and persistent glomerular filtration impairment was an indication for the verification of the diagnosis and identification of rapidly progressive glomerulonephritis based on a biopsy examination. Prompt diagnosis and inclusion of combined immunosuppressive therapy provided the chance to preserve renal function.