The present study deals with qualitative und quantitative analysis of osteoclastic bone resorption in metastatic bone disease. 267 cases were examined histomorphologically and divided into three developmental stages. In the first 'phase of early appearance' no bone resorption takes place. The stimulation of osteoclastic resorption in the surroundings of tumour tissue is typical in the second 'phase of interaction'. Pressure atrophy, aseptic necrosis and osteolysis by the tumour cells themselves are other mechanisms of bone destruction in the last 'phase of carcinomatosis'. Because osteoclasts are exclusively responsible for the loss of bone tissue in the 'phase of interaction', this stage is suited for precise quantitative analysis of osteoclastic resorption. 24 pure osteolytic secondary bone tumours of various primary lesions were examined histomorphometrically. The numerical values were compared with each other and with standard values of healthy individuals. In contrast with normal bone tissue the fractional resorption surfaces und osteoclast indices increase in metastases. Activated osteoclasts are larger and have more nuclei. The numbers of osteoclast index and nuclei per osteoclast are significantly higher in renal than in breast carcinoma. Osteoclasts can be activated in distances of more than 500 micron from tumour tissue. The mean stimulation distance in metastasis from squamous cell carcinoma is markedly higher than in secondary bone tumours of breast carcinoma. Several osteoclast activating substances and divers mechanisms of stimulation might be responsible for different numerical values of morphometric parameters in metastases from various primary malignancies.