Ozone is a potential alternative to chlorine and proprietary chemicals for treating cooling tower circulating water. This study surveyed selected air-conditioning cooling tower systems which use ozone treatment. The performance of these systems and the implications of using ozone treatment in large-scale industrial cooling systems were examined. The investigation indicated that several air-conditioning systems using ozone as the sole source of treatment are performing satisfactorily with respect to the control of biofouling, corrosion and chemical scaling. These systems are operating at very high cycles of concentrations (30 to 50 cycles). Similar performance in large-scale industrial cooling systems would produce significant economic and environmental benefits. The failure of one air-conditioning cooling tower system operating at elevated water temperatures suggests the technology must be better understood before it can be applied to large-scale industrial systems. A forthcoming research program is described. The program, which includes pilot testing, is designed to determine if the unexpected and unusual performance obtained in the small-scale air-conditioning systems using ozone can be extended to general industrial cooling, where cooling tower makeup is filtered secondary municipal effluent.