In a prospective survey in the Isle of Elba, 413 dogs affected by naturally acquired Leishmania infantum infection were identified out of a controlled population of 1,500 resident mongrel dogs. In all the 34 randomly selected dogs of different breed, age, and duration of disease, the presence of glomerular lesions which defined mainly two categories of glomerulonephritis (GN) was observed. The first group was characterized by mesangial-cell proliferation either with focal features (11 dogs), or with a diffuse pattern (10 dogs). The second group (12 dogs) showed the typical findings of segmental membrano-proliferative GN; amyloid deposits were seen in the glomerular tuft and interstitium in 1 dog. Immunohistochemical investigation revealed granular deposits of IgG, IgM, and C3 both in mesangial areas as well as on glomerular capillary walls. Granular immune deposits om the tubular basement membrane were also found in 31 out of 34 dogs examined. With ultrastructural investigation, subendothelial and mesangial electron-dense deposits were revealed. Age, sex, serum creatinine, BUN, duration of disease, anti-Leishmania antibody titers, and immune complexes did not discriminate between the types of observed GN, while proteinuria did. The study shows that the renal involvement is the natural sequela in dogs infected with L. infantum, and that the kidney lesions are characterized by immunologically mediated glomerular and tubular damage.