This article describes the development of the Checklist for Interpersonal Pain Behavior (CHIP), an observation scale which assesses overt pain behavior. The study is an extension of an earlier study [29] in which the dimensions and components of observed chronic pain behavior were examined. A broad definition of pain behavior is chosen (interpersonal pain behavior), namely the interaction between the pain patient and his/her direct environment. The list of pain behaviors, taken from the earlier study, has been transformed into a 78-item global rating scale to be used by nurses to quantify observed pain behavior in a clinical setting. Six studies examine the factor structure and the psychometric properties of this behavioral observation method. In the first study, 6 internally reliable factors are derived using factor analytic techniques from a sample of 152 chronic pain patients. They are labeled as: 'distorted mobility,' 'verbal complaints,' 'non-verbal complaints,' 'nervousness,' 'depression' and 'day sleeping.' Internal consistency of all factors, except 'day sleeping' was excellent. The following studies show that the CHIP is sufficiently reliable and valid. After a discussion on the advantages of this observation scale, the conclusion seems justified that the CHIP is a useful tool in pain assessment that can easily be used by nurses.