The preventive behaviour relating to visits for dental and medical examination of 100 mothers of pre-school children was investigated. The theory of reasoned action of Fishbein and Ajzen was used to predict the intention of 100 mothers to visit doctors and dentists. The study was conducted in one of the most socially deprived wards of the London Borough of Bloomsbury. The relationship between beliefs and attitudes held by mothers on preventive dental and repeated medical visits was investigated. In younger mothers aged 16-24 years, individual attitudes and subjective norms were better predictors of intention to visit the dentist every 6 months than were the total attitude and total subjective norm scores. The prediction of intention to visit the doctor and the dentist showed a similar pattern, but the variance accounted for by intention to visit the doctor was lower. Younger mothers showed stronger attitudes associated with dental treatment outcome, whereas older mothers showed a more positive preventive dental orientation. This finding is supported by principal components analysis of both the medical and dental data. The theory of reasoned action was successfully applied, but the external variable age was a very important prediction factor. This modifying factor, in relation to Ajzen and Fishbein's theory, gives a better insight into preventive dental and medical health behaviours.