Cigarette brands that deliver < or = 15 mg of tar in official smoking-machine tests accounted for 72.7% of total cigarette sales in 1995. Many of these brands use ventilated filters-a system with small perforations around the filter that are designed to draw in additional air during smoking. In brands with ventilated filters, air introduced through the vents dilutes the amounts of tar, nicotine, carbon monoxide (CO), and other hazardous constituents of cigarette smoke. This report summarizes results of tests conducted by researchers at The Pennsylvania State University during July 1997 to measure the percentage of air drawn through the filter vents of 32 brands of U.S. cigarettes that have tar yields rated by the Federal Trade Commission (FTC) as ranging from 1 mg-18 mg; the report also examines the correlation between the degree of filter ventilation and tar yield. The findings indicate that 30 (94%) of 32 brands tested were ventilated and that percentage filter ventilation varied inversely with standard tar, nicotine, and CO yields.