The purpose of this study was to determine the concentrations of Haptoglobin (Hp), C-reactive protein (CRP) and Fibrinogen (Fib) in dogs with spontaneous hypercortisolism (HCT), diabetes mellitus (DM) and hypothyroidism. Stored (-20°C) serum (Hp and CRP) or citrate plasma (Fib) samples from 35 dogs with HCT (31 PDH and 4 FAT), 24 with DM and 7 with hypothyroidism were analyzed. All samples were obtained from newly diagnosed dogs before starting the therapy for the specific endocrinopathy. Reference ranges for APPs were previously obtained from a population of 25 clinically healthy dogs. CRP and Hp were measured using human immunoturbidometric assays validated in our laboratory for the dog, as previously reported. CRP concentrations (reference range 0-0.5 mg/dl) were between 0.01 and 1.63 (median 0.01; abnormal in 14.3% of cases), between 0.01 and 11.60 (median 1.03; abnormal in 62.5% of cases), and between 0.01 and 9.82 (median 0.01; abnormal in 42.9% of cases) in HCT, DM an hypothyroid dogs respectively. Hp concentrations (reference range 20-140 mg/dl) were between 0 and 590 (median 276; abnormal in 82.9% of cases), between 61 and 387 (median 152; abnormal in 62.5% of cases), and between 2 and 242 (median 109; abnormal in 42.9% of cases) in HCT, DM an hypothyroid dogs respectively. Fib concentrations (reference range 1.45-3.85 g/l) were between 0.82 and 6.16 (median 3.77; abnormal in 47.1% of cases), between 2.32 and 4.27 (median 3.63; abnormal in 41.7% of cases), and between 3.31 and 6.69 (median 4.40; abnormal in 71.4% of cases) in HCT, DM an hypothyroid dogs respectively. CRP concentrations were significantly lower (p<0.001) in HCT compared to DM dogs. Hp concentrations were significantly higher in HCT compared to DM (p<0.001) and hypothyroidism (p=0.003). Fib was higher in dogs with hypothyroidism compared to dogs with DM but not statistically significant (p=0.051). Only 5 dogs with HCT had a (mild) increase of CRP: 2 had also a severe pyoderma, 2 concomitant DM with ketoacidosis and 1 concomitant mediastinal tumor and haemolytic anaemia. In these cases we considered inadequate the increase of CRP when compared to the expected acute phase response. The high serum concentrations of Hp in dogs with HCT has already been observed and our results are comparable to those reported in the literature. Lack of exact knowledge regarding the inflammatory/infectious status of each dog is the main limitation of this study. In conclusion APPs were high in a large number of dogs with endocrinopathies and this is probably due to the high incidence of concomitant infectious diseases. CRP is low in dogs with HCT and, like Hp, in this endocrinopathy should be considered a poor marker of the acute phase response. Further studies are required to assess whether serum Hp and CRP could be useful for the diagnostic protocol of dogs with HCT.