Patient self-reports are the primary method for capturing the experience of pain, and diaries are often used to collect patient self-reports. This study was designed to determine if momentary monitoring of pain with an electronic diary affected pain levels over time, if it affected weekly recall of pain, and if daily sampling density affected compliance rates and patients’ reactions to the study. Ninety-one patients with chronic pain were randomized into four groups with differing levels of momentary monitoring over 2 weeks. Little support was found for reactivity defined as temporal shifts in pain over the study or as changes in recalled weekly pain due to momentary monitoring. Compliance with the electronic diary protocol was 94% or better, and was not related to sampling density. Patients reported little difficulty with the diary procedures and were not unduly burdened by the protocol.