This paper reports two studies of chronic pain patients (n = 444) relevant to the psychometric properties of the Pain Disability Index (PDI), a self-report instrument that has been used to assess the degree to which chronic pain interferes with various daily activities. In the first study, patients with high PDI scores reported more psychological distress (P less than 0.001), more severe pain characteristics (P less than 0.001), and more restriction of activities (P less than 0.001) than patients with low PDI scores, findings supportive of the construct validity of the measure. Further, a multiple regression showed that a linear combination of 9 variables predicted PDI scores (R = 0.74): time spent in bed, psychosomatic symptoms, stopping activities because of pain, work status, pain duration, usual pain intensity, quality of life, pain extent, and education. This study also showed differences for age and gender on disability. The second study involved 46 patients who had undergone inpatient treatment for their pain conditions. The study revealed modest test-retest reliability for the instrument. It also showed the PDI to be associated with the levels of pain behavior exhibited by these patients. The findings of both studies generally support the reliability and validity of the PDI as a brief measure of pain-related disability. Questions regarding its test-retest reliability and lack of association with certain pain behaviors are discussed, as are suggestions for future research.