This study aimed to use a population-based sample to assess the psychological validity of the Seattle system for diagnosing dentally anxious individuals. This system consists of four diagnostic types in which such individuals are categorized according to the main source of their fear regarding dental treatment. Subjects were 1420 randomly selected adults aged 18 years and over who took part in a two-stage mail survey. The questionnaires contained measures of dental anxiety and standardized measures of general anxiety and fearfulness. Allocation to the Seattle categories was based on responses to four diagnostic items. Overall, 16.4% of the sample were dentally anxious. Their distribution across the four Seattle types was as follows: type I (simple conditioned phobia)--49.6%; type II (fear of catastrophe)--7.8%; type III (generalized anxiety)--19.4% and type IV (distrust of dentists)--9.9%. The remaining 13.3% could not be categorized. Judging by their scores on measures of dental anxiety, these subjects were borderline cases. Scores on the measures of anxiety and fearfulness indicated that the diagnostic system was valid and identified sub-groups of the dentally anxious population which were internally consistent. However, all subjects indicated extreme fear of dental treatment and were broadly similar in terms of their cognitive and behavioral responses to dental care. Of particular interest was the distribution of diagnoses according to age. Younger subjects were more likely to be categorized as type I, while older subjects were more likely to be categorized as type III. Overall, the results indicate that dental anxiety is a complex fear with a number of components.