Comment: 40 pages, 13 figuresTwo decades of studies have found significant regional differences in the timing of transitions in national business cycles and their durations. Earlier studies partly detect regional synchronization during business cycle expansions and contractions in Europe, the United States, and Japan. We examine this possibility applying a sophisticated method for identifying the time-varying degree of synchronization to regional business cycle data in the U.S. and Japan. The method is prominent in nonlinear sciences but has been infrequently applied in business cycle studies.We find that synchronization in regional business cycles increased during contractions and decreased during expansions throughout the period under study.Such asymmetry between the contraction and expansion phases of a business cycle will contribute our better understanding of the phenomenon of business cycles.