Under-reporting tobacco consumption is common, although there is lack of evidence whether under-reporting is associated with health risk perception. In this study, smoking topography from 110 smokers was recorded over 24 hours, aiming to capture a representative average of smoking behaviour. Participants significantly under-reported the duration of secondary exposure, and there was a significant interaction between self-report distortion type and risk perception. Analysis showed that smokers under-reporting puff number declared perceiving significantly less susceptibility to acquiring airway diseases, which is correlating significantly with the level of under-reporting. The present findings may suggest that under-reporting smoking behaviour have psychological functions beyond achieving social desirability.