This paper reports the development of a self-report instrument designed to assess pain in cancer and other diseases. It is argued that issues of reliability and validity should be considered for every pain questionnaire. Most research on measures of pain examine reliability to the relative neglect of validity concerns. The Wisconsin Brief Pain Questionnaire (BPQ) is evaluated with regard to both reliability and validity. Data from patients with cancer at 4 primary sites and from patients with rheumatoid arthritis suggest that the BPQ is sufficiently reliable and valid for research purposes. Additional methodological and theoretical issues related to validity are discussed, and the need for continuing evaluation of the BPQ and other measures of clinical pain is stressed.