Background. Therapists frequently advise the use of activity pacing as a coping strategy to manage long-term conditions (eg, chronic low back pain, chronic widespread pain, chronic fatigue syndrome/myalgic encephalomyelitis). However, activity pacing has not been clearly operationalized, and there is a paucity of empirical evidence regarding pacing. This paucity of evidence may be partly due to the absence of a widely used pacing scale. To address the limitations of existing pacing scales, the 38-item Activity Pacing Questionnaire (APQ-38) was previously developed using the Delphi technique. Objective. The aims of this study were: (1) to explore the psychometric properties of the APQ-38, (2) to identify underlying pacing themes, and (3) to assess the reliability and validity of the scale. Design. This was a cross-sectional questionnaire study. Methods. Three hundred eleven adult patients with chronic pain or fatigue participated, of whom 69 completed the test-retest analysis. Data obtained for the APQ-38 were analyzed using exploratory factor analysis, internal and test-retest reliability, and validity against 2 existing pacing subscales and validated measures of pain, fatigue, anxiety, depression, avoidance, and mental and physical function. Results. Following factor analysis, 12 items were removed from the APQ-38, and 5 themes of pacing were identified in the resulting 26-item Activity Pacing Questionnaire (APQ-26): activity adjustment, activity consistency, activity progression, activity planning, and activity acceptance. These themes demonstrated satisfactory internal consistency (Cronbach a=.72-.92), test-retest reliability (intraclass correlation coef-ficient=.50-.78, P<.001), and construct validity. Activity adjustment, activity progression, and activity acceptance correlated with worsened symptoms; activity consistency correlated with improved symptoms; and activity planning correlated with both improved and worsened symptoms. Limitations. Data were collected from self-report questionnaires only.Conclusions. Developed to be widely used across a heterogeneous group of patients with chronic pain or fatigue, the APQ-26 is multifaceted and demonstrates reliability and validity. Further study will explore the effects of pacing on patients' symptoms to guide therapists toward advising pacing themes with empirical benefits.