BACKGROUND The rates of lung adenocarcinoma cancer have risen more rapidly than the rates of lung squamous cell cancer over the past 2 decades. METHODS A case-control study was carried out to assess the impact of long-term filter cigarette smoking on the risk of squamous cell carcinoma (SCC) and adenocarcinoma (AC) of the lung. RESULTS Odds ratios for SCC among subjects who had smoked only filter cigarettes were reduced relative to lifetime nonfilter cigarette smokers by 30% for men and by 60% for women, but no risk reduction was observed for AC of the lung. CONCLUSIONS The predominance of AC over SCC may be due in part to the fact that smokers of very low yield cigarettes tend to compensate for the lower nicotine levels by inhaling more deeply and frequently, leading to greater exposure of the peripheral lung to the carcinogens in tobacco smoke, and in part to the increased concentration of nitrosamines that preferentially produce AC in laboratory animals.