Type 1 diabetes mellitus is the most common metabolic disease in childhood, which may, after years, lead to late vascular complications, including diabetic kidney disease. Microalbuminuria, which progresses to evident proteinuria, is the first sign of this disease. However, other causes, such as nondiabetic nephropathies, should be also included in the diagnosis in diabetic patients. This is particularly important in cases where disease duration by the time proteinuria develops is short, and a good control of glycemia is maintained. We present a case of a 9-year-old girl with type 1 diabetes mellitus who was diagnosed with increasing proteinuria and a developing nephrotic syndrome at 16 months after diabetes diagnosis. The histopathological picture of renal biopsy specimen corresponded to minimal change disease. Steroid therapy was initiated with simultaneous adjustment of insulin doses, which allowed for complete clinical and biochemical remission of nephropathy with maintained normal glycaemia. The girl is under the care of a diabetologist and a nephrologist. In conclusion, a coexisting nephropathy should be considered in the case of proteinuria in a child with diabetes mellitus.