The presence of parathyroid hormone related protein (PTHRP) in human breast cancers has been assessed by immunohistochemistry using a polyclonal antiserum specific for the mid-region sequence 37-67 in an immunoperoxidase technique. The primary tumours from 155 normocalcaemic, consecutive women with early breast cancer who had been followed up for a minimum of 5 years were assessed. Dewaxed paraffin sections of formalin fixed tissue was used throughout. Positive PTHRP staining was detected in 56% of the cancers and was unrelated to standard prognostic factors, recurrence or survival. However, PTHRP positivity was related to the development of bone metastases (P less than or equal to 0.03) and hypercalcaemic episodes. PTHRP is implicated as the humoral factor responsible for hypercalcaemia associated with breast cancer and tumour positivity may be a useful predictor of which women will develop bone metastases.