The occurrence of symptoms of nephritic syndrome in the form of oliguria, arterial hypertension, proteinuria and haematuria is routinely interpreted as acute post-infectious glomerulonephritis. The two clinically similar cases of nephritic syndrome indicate the need for a differential diagnosis in order to identify the correct condition and establish appropriate therapy. In the first case, the development of nephritic syndrome was preceded by pharyngitis that had occurred 2 weeks before. A decrease in a concentration of complement components was accompanied by an increased antistreptolysin O titre. A diagnosis of acute post-streptococcal glomerulonephritis enabled the implementation of symptomatic treatment involving diuretics and hypotensive agents. Clinical improvement was seen in the second week of treatment. In the second case, there was no correlation with an infectious episode. A decrease in a concentration of complement components was accompanied by thrombocytopenia and the presence of antinuclear antibodies. This configuration of signs necessitated renal biopsy, the results of which enabled correct diagnosis of lupus nephritis and implementation of combined immunosuppressive treatment that gave a chance for preserving renal function.