Results from a prospective illness-absence surveillance of refinery and petrochemical workers from 1986 through 1994 are presented. Illness absence data for this study were extracted from the morbidity section of the Shell Oil Company's Health Surveillance System, which includes records of all illness absences in excess of 5 days. The majority of employees (59%) had no illness absence during the 9-year period studied. The 13% of the population who had three or more absences accounted for 63% of the total illness absence episodes and 62% of the total work days lost. Frequency rate and duration of absence increased with increasing age. The increased illness absence was associated with the presence of known health risk factors, such as smoking, elevated blood pressure, high cholesterol, and obesity. For example, obese women had a twofold increased illness absence rate compared with nonobese women and the rate for male smokers doubled that of nonsmoking men. These health risk factors are also more common among employees with three or more absences than those with fewer or no absences. The goal of this analysis is to quantify the impact of illness absence to develop disease prevention strategies to maximize good health in employees and to minimize both the frequency and duration of illness absence.