Japanese lesson study, in various adapted forms, is becoming increasingly significant in professional development of mathematics teachers in the USA. Our goal in the research reported in this paper was to investigate, in a three-tiered teaching experiment, the cycles of learning of two researchers, six teachers, and the students in three grade 4 classes and three grade 5 classes, in a six-month long lesson study project in the first half of 2002. The learning processes evolved in each of the three tiers (students, teachers, and researchers) over the course of three iterations of a lesson on measurement taught respectively in grades 4 and 5 by the three grade 4 and three grade 5 teachers. This paper documents some of these shifts in learning in each of the three tiers and assesses this form of lesson study for professional development, through the eyes of the teachers. (Contains 2 footnotes, 1 figure and 2 tables.) [For complete proceedings, see ED500860.]