The biological implications of serum and tissue prolactin levels in canine mammary tumours (CMT) have been previously described although the influence of this hormone on inflammatory mammary carcinomas as well as its value as prognostic indicator remains to be properly clarified. Prolactin determinations were carried out by enzyme immunoassay in tumour tissue and serum of 39 female dogs with spontaneous CMT and in normal mammary gland and serum of 10 controls. Prolactin levels were higher in the case of CMT compared to controls (P<0.05). In malignant CMT, higher levels of tissue prolactin were associated with the occurrence of tumour relapse and/or distant metastasis (P<0.05). Inflammatory mammary carcinomas presented the highest values for tissue prolactin concentrations with concentrations significantly higher than other malignant non-inflammatory mammary carcinoma tumours (P<0.05). The high levels of prolactin found in cases with poor clinical prognoses, including inflammatory mammary carcinoma, open the possibility of being able to better stratify clinical cases in malignant CMT with a view to tailoring treatment appropriately.