The incidence of osteosarcoma in Northern Ireland was compared with that in the Republic of Ireland to establish if differences in incidence between the two regions could be related to their different drinking water fluoridation policies. Data from the Northern Ireland Cancer Registry (NICR) and the National Cancer Registry of Ireland (NCRI) on osteosarcoma incidence in the respective populations were used to estimate the age-standardised and age-specific incidence rates in areas with and without drinking water fluoridation. One hundred and eighty-three osteosarcoma cases were recorded on the island of Ireland between 1994 and 2006. No significant differences were observed between fluoridated and non-fluoridated areas in either age-specific or age-standardised incidence rates of osteosarcoma. The results of this study do not support the hypothesis that osteosarcoma incidence in the island of Ireland is significantly related to public water fluoridation. However, this conclusion must be qualified, in view of the relative rarity of the cancer and the correspondingly wide confidence intervals of the relative risk estimates.