Background: The objective of this research was to develop a caregiver-reported clinical outcome assessment (COA) measure designed to assess observable behaviors of children, ages 4 to 12 years, with autism spectrum disorder (ASD) for supporting labeling claims of treatment benefit. Methods: Development of the measure included a review of the literature and existing instruments, conceptual disease model development, concept elicitation focus groups, item generation, and cognitive debriefing interviews. Results: Predominant characteristics and behaviors of ASD identified by the literature and instrument reviews included sociability, communication deficits, stereotypy, inattention and hyperactivity, irritability, anxiety, and familial impact. In each of the 10 instruments reviewed, evidence of content validity was limited or nonexistent. Predominant themes arose across 8 major categories during concept elicitation. A total of 27 concepts were identified through focus group feedback and formed the basis for item development and cognitive pre-testing. Revisions to the items yielded a final version of a daily diary containing 21 items assessing observable behaviors and characteristics of ASD in children 4 to 12 years old. Conclusions: The Observable Behaviors of ASD Scale (OBAS) was developed as a self-administered, caregiver-reported measure containing 8 predominant themes. Items are scored on one of two 5-point ordinal categorical response scales, and the recall period for each item is the past 24 hours. This research provides evidence that the OBAS is content valid for assessing treatment benefit, which was found to be lacking in other instruments.