The aim of this study was to determine C-reactive protein levels in dogs with benign and malignant mammary tumors.Thirty female dogs, with ages ranging between 6 and 15 years and with no distinction of breed were used for this purpose.The animals were divided into 3 different groups of 10 dogs each: Group 1 (control), Group 2 (benign mammary tumor) and Group 3 (malignant mammary tumor).The neoplasias were classified histologically and C-reactive protein (CRP) levels were analyzed using a human turbidimetric immunoassay validated for dogs.Group 3 had the highest values (mean: 8.2 mg/L; median: 7.1 mg/L) of C-reactive protein compared with the other groups (P < 0.05).According to a discriminant analysis, a female dog with a mammary tumor and CRP values ≥ 8 mg/L has a 61% or greater probability of this tumor being malignant.