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 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.