This study investigated the effectiveness of an Internet-based intervention for people with panic disorder. Twenty-two participants met criteria for panic disorder and were randomly assigned to either the treatment or a self-monitoring control condition. The study took place over 3 weeks and consisted of one week of self-monitoring prior to the intervention group accessing the program for one week, followed by one week of post-intervention assessment. Participants were assessed on measures pertaining to panic, negative affect, body vigilance, anxiety sensitivity and self-efficacy in managing panic. The treatment condition was associated with significant reductions in all variables except anxiety sensitivity and depressive affect. This type of intervention for people with fearful panic attacks holds promise as an efficacious and economical method for treating spontaneous panic.