Studies have postulated a role for cognitive ability in socio-economic inequalities in general health. This role has not been examined for oral health inequalities. We examined whether cognitive ability was associated with oral health, and whether it influenced the relationship between oral health and socio-economic position. Data were from the Third National Health and Nutrition Examination Survey (1988-1994), for participants aged 20-59 years. Oral health was indicated by extent of gingival bleeding, extent of loss of periodontal attachment, and tooth loss. Simple reaction time test, symbol digit substitution test, and serial digit learning test indicated cognitive ability. Education and poverty-income ratio were used as markers of socio-economic position. Participants with poorer cognitive ability had poorer oral health for all indicators. The association between oral health and socio-economic position attenuated after adjustment for cognitive ability. Cognitive ability explained part, but not all, of the socio-economic inequalities in oral health.