The Health Belief Model (HBM; Rosenstock, 1966) is the most widely used psychological theory of health-related behaviours but its applicability to non-Western, especially Asian, populations has not been systematically studied. The main objective of the current study was to examine the effects of components of the HBM and dispositional optimism on preventive intention in two separate samples of 220 Hong Kong Chinese adults and 340 Hong Kong Chinese adolescents. Two HBM variables, benefits and barriers, were experimentally manipulated for an imaginary flu outbreak and a hypothetical vaccine. Optimism in the two samples was assessed using the Chinese revised Life Orientation Test (Lai et al., 1998). Participants indicated their intention to take the hypothetical vaccine after reading information for each of the treatment combinations denned by the two HBM variables. The order of the presentation of each of the six treatment conditions was counterbalanced. Results of ANOVA showed that the effects of the two HBM variables were similar in the two groups. Higher benefits and lower barriers were associated with stronger behavioural intention to take the vaccine. The two HBM variables interacted significantly in the adolescent but not the adult sample. Among the adolescents, the difference in intention scores between low and high barriers became larger when the benefits of taking the vaccine increased. In addition, optimism had significant effect on preventive intention only among the adolescents. These findings suggest that age may be an important factor moderating the effects of HBM components as well as optimism on preventive intention in Hong Kong Chinese. Implications of these findings to future research among Chinese were discussed. Copyright © 2004 International Union of Psychological Science.