Organic anion transporting polypeptides (OATP) were identified as transmembrane transporters for various endo- and exogenous organic compounds (hormones, prostaglandins, anticancer drugs). OATP expression had been shown in different tissues, but not in bone tumors. Therefore, the expression pattern of all known eleven human OATPs was analyzed by quantitative RT-PCR in 21 human bone tumor specimens (osteosarcomas, bone metastases and benign aneurysmal bone cysts). Transcriptional expression of OATP1A2, 1C1, 2A1, 2B1, 3A1, 4A1, 4C1 and 5A1, but not of OATP1B1, 1B3 and 6A1 was observed in malignant and non-malignant tumor specimens at varying level. Importantly, OATP3A1, 4A1, 2B1 and 1C1 mRNA levels were significantly higher in aneurysmal bone cysts as compared to osteosarcomas. Elevated mRNA levels of OATP2A1, 1A2, and 4C1 in metastases from kidney cancer and of OATP5A1 in prostate cancer suggest that the OATP expression pattern in metastases is comparable to that of the primary tumors. Different to tissue, OATP expression in osteosarcoma cell lines HOS and MG-63, normal human osteoblast outgrowth cells (hOB) and bone marrow stromal cells (BMSC) is limited to OATP3A1 and OATP4A1. High OATP expression levels, particularly in benign bone tumors, suggest an important role of these transporters for providing hormones, their conjugates, prostaglandins and drugs in bone cells. Thereby, they may influence bone resorption and formation.