We examined illness patterns in a cohort of 530 male and female runners who completed a monthly log for 12 months. The average number of upper respiratory tract infections (URTIs) per person per year for the cohort was 1.2. An upper respiratory tract infection was indicated by the report of any of the following symptoms; runny nose, sore throat, or cough. Using a multiple logistic regression model, the following factors were found to be associated with having one or more URTIs in the follow-up period: living alone (odds ratio = 2.27, 95% CI = 1.01, 5.09), running mileage (486-865 miles, odds ratio = 2.00, 95% CI = 1.01, 2.78; 866-1388 miles, odds ratio = 3.50, 95% CI = 1.52, 4.44; greater than 1388 miles, odds ratio = 2.96, 95% CI = 1.30, 3.68), body mass index greater than the 75th percentile (odds ratio = 0.58, 95% CI = 0.35, 0.94), and male gender (odds ratio = 0.14, 95% CI = 0.03, 0.68). A significant interaction was found to exist between gender and alcohol use, with the association between alcohol use and upper respiratory tract infections being positive in males and negative in females. These results suggest that running dosage (mileage) is a significant risk factor for upper respiratory tract infections in this group of exercisers.