Background Many people suffer from subthreshold and mild panic disorder and are at risk of developing more severe panic disorder. Aims This study (trial registration: ISRCTN33407455) was conducted to evaluate the effectiveness of an early group intervention based on cognitive-behavioural principles to reduce panic disorder symptomatology. Method Participants with subthreshold or mild panic disorder were recruited from the general population and randomised to the intervention (n = 109) or a waiting-list control group (n = 108). The course was offered by 17 community mental health centres. Results In the early intervention group, 43/109 (39%) participants presented with a clinically significant change on the Panic Disorder Severity Scale-Self Report (PDSS-SR) v. 17/108 (16%) in the control group (odds ratio (OR) for favourable treatment response 3.49, 95% Cl 1.77-6.88, P=0.001). The course also had a positive effect on DSM-IV panic disorder status (OR = 1.96, 95% C1=1.05-3.66, P=0.037). The PDSS-SR symptom reduction was also substantial (between-group standardised mean difference of 0.68). The effects were maintained at 6-month follow-up. conclusions People presenting with subthreshold and mild panic disorder benefit from this brief Intervention.