Breast cancer and cartilaginous tumours (enchondroma and chondrosaroma) were found to occur rather frequently in the same patient. In order to identify a possible association between occurrence of these tumour types, a population-based study was performed. This was a nation-wide case-control study, using the Dutch national pathology database. Between 1973 and 1998, the study identified 2295 cases of cartilaginous tumours in female patients and 132 636 females with breast cancer. Of these patients, 61 were diagnosed with both tumour types. To exclude a possible bias due to screening for occult bone metastases in breast cancer patients, a similar analysis was performed for lung cancer, since screening is performed similarly in lung cancer patients. Of 16 559 females diagnosed with lung cancer, only one case with a cartilaginous tumour was found. The odds ratio for a potential association of breast and cartilaginous tumours is 7.62, implicating a 7.62 increased risk for the same female patient having both breast cancer and a cartilaginous tumour. Furthermore, the mean age of onset in patients with breast cancer as the first tumour is nearly 10 years earlier than breast cancer in general, i.e. 51 years versus 60.9 years. The association of breast cancer and cartilaginous tumours and the early age of onset of breast cancer in these patients may suggest a genetic trait.