OBJECTIVES: In acute pain studies, subjects are asked to report on symptoms at specific intervals after dosing. One or more assessments are often primary endpoints. When timed assessments are collected on paper the actual time the assessments were completed is unknown, and enforcing completion of the assessment at a specific time is impossible. Electronic patient reported outcome technologies (ePRO) allow control over the window in which a subject can complete the electronic diary (e-Diary), and a time stamp associated with diary completion. E-Diaries assure the investigator of more reliable information: the 120 minute assessment was completed at or near 120 minutes. But is there a down side? With a restricted window of time for completing an e-diary in the acute pain model, how compliant will subjects be?  METHODS: Diary completion was examined in 12 randomized clinical trials using the acute pain model. Indications were surgical pain, migraine, or break through pain. Subjects completed pain diaries at timed intervals after dosing. In 4 trials, subjects completed at least some of the assessments in the clinic or post surgical area allowing comparison of compliance in supervised settings to compliance with e-diaries at home. Frequency of the assessments varied across the trials, allowing some description of factors which may influence compliance. Finally, design features, like a reminder alarm, were correlated with time of diary compliance to understand the usefulness of these features.  RESULTS: e-Diary compliance in supervised setting can be as high as 100%. Compliance in unsupervised settings is typically 80%, although frequent assessments correspond with decreased compliance. In one study that required extensive frequent assessments, e-Diary compliance is much lower. Indication does not correlate with e-Diary compliance in unsupervised settings. CONCLUSIONS: Factors that correspond with e-diary compliance are supervision, frequency and overall demand on subjects.