In assessments of the cost of illness, productivity losses potentially constitute a large proportion. The present study objective was to develop a method to measure restricted days and to quantify total productivity loss days (PLDs) in adult asthma patients. Patient and disease characteristics, occupation, annual wage, work absences, restricted days, level of functioning on restricted days, and travel and waiting time were collected over 6 months in 892 adult asthma outpatients residing in southern Ontario. Annual PLDs varied from 12 in employed persons to 49 in disability pensioners. Homemakers reported 22 PLDs per year. Restricted days accounted for most PLDs and functional level during restricted days varied from 55% to 81%. Annual PLDs increased with increasing disease severity. Employed persons experienced the fewest PLDs and functioned at the highest level during restricted days, but also demonstrated a milder disease compared with other groups. Most productivity loss in asthma patients resulted from numerous restricted days, a category of PLD that is often ignored in economic assessments. The presentation of PLD results disaggregated by category of time loss and wage rate may provide valuable information to employers and health policy makers and may facilitate the application of multiple approaches to the calculation of indirect costs.