OBJECTIVES: To validate the Work Productivity and Activity Impairment questionnaire (WPAI-asthma) that measures self-reported absence from work, reduction in productivity while at work, and reduction in productivity while doing regular activities, other than work. METHODS: The WPAI-asthma questionnaire and the generic health-related quality of life questionnaire SF-36 were adapted for use over the Internet and administered among 1272 asthmatics from Denmark, Norway and Sweden, in a web-based survey. In addition to the questionnaires, the individuals were asked to report what asthma medication they had been prescribed and how much health care they had used the previous month because of their asthma. Demographic data were also collected. Individuals were classified into different disease severity based on medication use. Productivity variables derived from the WPAI-asthma were correlated to quality of life dimensions, age, and health care resource use. Furthermore, the relationship between productivity variables and disease severity was investigated. RESULTS: High correlation coefficients (defined as > 0.4) were found between productivity while working and while doing regular activities and quality of life dimensions related to work and daily activities. A low correlation (defined as <0.2) was found between self-reported productivity and age. On average, the employed asthmatics (n = 785) reported an average of 2.4% of their time absent from work due to asthma symptoms. Productivity while working was reduced by 10.1% and the reduction in productivity while doing regular activities was 13.2%. Unemployed asthmatics (n=397) only reported reduction in productivity while doing regular activities, and the mean reduction was 22.5% due to asthma symptoms. Asthmatics with a more severe disease status reported higher productivity impairment than did those with a milder disease (p <0.001). CONCLUSIONS: The results indicate that the WPAI-asthma has a high convergent and discriminate validity.