This study evaluated the effect of claustrophobia, an abnormal dread or fear of closed spaces, on adherence to continuous positive airway pressure (CPAP) therapy. The design was a secondary analysis of data from a prospective study of participants (N = 153) that completed 3 months of CPAP therapy from seven sleep disorders centers in the United States and Canada. A 15-item subscale adapted from the Fear and Avoidance Scale measured claustrophobic tendencies pre-CPAP treatment and again after 3 months. An overt monitor attached to the CPAP machines recorded mask-on CPAP adherence. There was a statistically significant difference in claustrophobia scores by adherence group (< 2 hours, 2 to < 5 hours, > or = 5 hours) and time period (pre-CPAP and after 3 months CPAP). Poor CPAP adherence (< 2 hours per night)was more than two times higher in participants with a claustrophobia score > or = 25. Identification of persons with increased claustrophobia tendencies and targeted interventions may increase adherence.