AIM--To assess parathyroid hormone related protein (PTHrP) as a candidate biochemical marker of invasion of the mandible by oral squamous cell carcinoma. METHODS--Tumour PTHrP concentrations were quantitated by immunoassay, and PTHrP was detected by immunohistochemistry, in a cohort of 24 primary squamous cell carcinomas of the mandible. RESULTS--PTHrP was identified in all tumours examined, but no correlation was found between scores of the intensity and/or consistency of staining or tumour PTHrP concentrations and the histological classification of tumour invasion. CONCLUSION--Although PTHrP was present in all squamous tumours studied, there was no correlation between PTHrP expression and pattern of tumour invasion. However, tumour derived PTHrP may act locally to influence tumour growth and differentiation and resorption of bone.