This study examined the effects of lesson study on participants' classroom observations and perceptions of lesson effectiveness, by investigating the focus of their observations during a mathematics lesson and their ratings of the lesson's effectiveness, both preceding and subsequent to the lesson study experience. Prior to the lesson study, subjects viewed a videotaped lesson, rated the lesson's effectiveness on a scale of 1 to 10, and provided a detailed rationale for their ratings. After the lesson study experience, the process was repeated. Pre- and post-lesson study ratings were compared, and comments were categorized as relating to teacher behavior, student behavior, or other. Analyses revealed that the focus of participants' observations remained relatively constant, while perceptions of lesson effectiveness, particularly with regard to teacher instruction, showed some change. Implications of the study and questions worthy of further exploration are discussed.