The present study examined the reliability and validity of a daily activity diary for chronic pain patients. The diary assesses various postures and activities including time spent lying, sitting, or standing/walking, the use of pain relief devices, time spent in pain relief activities, and the use of analgesic medications. The methodology compared patient self-report on the daily activity diary to spouse observations of the same activities. In addition, patient self-report of uptime/downtime was compared to the objective assessment of uptime/downtime by an automated electromechanical device. Reliability coefficients for the daily activity diary categories were all positive and statistically reliable as were correlations between patient and spouse ratings on lying down time, time spent standing/walking, and pain intensity. Also, patient self-report of medication use correlated significantly with spouse pill count. Finally, the correlation between patient report of lying down time and downtime as measured by the electromechanical monitor was also positive and highly significant. These results indicate that the daily activity diary is a reliable and valid instrument for the assessment of daily activity patterns of chronic pain patients in their natural environment. These results are discussed in relation to other research and the use of daily diaries for assessment and treatment outcome research with this population.