Light scattering pattern analysis (LSPA) was applied in the current study for accurate and sensitive detection of subtle changes in cell size, which occur in mouse thymocytes undergoing apoptosis. The decrease in cell diameter as measured by LSPA was found to be an early signal of apoptosis preceding the externalization of phosphatidylserine on the outer membrane. When apoptosis was induced by dexamethasone, the change in cell size was dose and time dependent, and could be blocked by pretreatment of the thymocytes with N-acetylcysteine (NAC). This implies that the scattering pattern, when combined with fluorescent markers such as annexine-V, may be a powerful tool for early detection of apoptosis. Another advantage gained by the use of this method is the ability to repeatedly trace the same cells and to monitor the kinetics of their size changes.