The association of cigarette smoking with the occurrence and severity of an acute respiratory tract illness (ARTI) was studied. Clinical data were obtained prospectively on 867 men and women with an ARTI and on a control group of 289 women. Three hundred seven (57%) of the 534 women in the ARTI group were smokers, compared with 97 (34%) of the 289 women in the control group, a highly significant difference. Of the 867 men and women with ARTIs, 506 were smokers. Smokers had a statistically significant greater likelihood of having a lower respiratory tract illness (57% v 45%), a longer duration of cough (8.9 v 6.8 days), and a greater frequency of abnormal auscultatory findings (31% v 20%) than did the 361 nonsmokers. Cigarette smoking may thus contribute to the substantial morbidity and lost productivity resulting from ARTI.