Ninety-three unhealthy dogs (including some with diabetes mellitus or insulinoma) of different ages, sex and breeds were divided into 10 groups according to their pathology. Serum fructosamine concentrations were determined using a commercial colorimetric nitroblue tetrazolium method. Diabetic dogs had the highest fructosamine concentrations (454.85 +/- 149.34 micromol/L). Dogs with insulinoma had significantly lower fructosamine concentrations (202.80 +/- 31.22 micromol/L), similar to those with leishmaniosis (202.83 +/- 99.83 micromol/L). Fructosamine concentrations in non-healthy dogs, except those with diabetes mellitus, insulinoma or leishmaniosis, were within the reference limits previously reported.