A 10-year-old boy was evaluated for fever, weight loss, uveitis, normocytic, normochromic anemia, renal insufficiency, and hypergammaglobulinemia of 8 weeks' duration. Infectious and neoplastic causes of fever were excluded. A renal biopsy performed in the 4th week of disease revealed diffuse plasmocytic interstitial nephritis. No treatment was prescribed and the patient was transferred to another hospital. Because clinical symptoms and renal insufficiency were still present, in the 8th week of disease a second biopsy was performed, which showed lympho-monocytic interstitial nephritis. At the same time, phenotypic analysis of peripheral blood mononuclear cells was carried out, revealing a significantly decreased number of CD3(+), CD4(+), and CD3(+)/CD8(+) cells, increased non-T CD3(-)/CD8(+) and CD56(+) NK cells, and decreased "naïve" (CD45RA(+)/CD4(+)) and memory (CD45RO(+)/CD8(+)) T lymphocytes. A 6-month course of oral prednisone was prescribed. Clinical symptoms and laboratory findings quickly returned to normal values. After 13 days of corticosteroid therapy, a second phenotypic analysis of peripheral blood mononuclear cells was performed, which revealed normalization of CD3(+), CD4(+), and CD3(+)/CD8(+) cells as well as proportions of non-T CD8(+) and CD56(+) NK lymphocytes, "naïve" and memory cells. This case shows spontaneous evolution of tubulointerstitial infiltrates from plasmacytic to lympho-monocytic, profound disturbances of the immunological system, and the beneficial effect of corticosteroids on both the clinical course and immunological disturbances.