OBJECTIVE: Asthma is a very common disease, affecting approximately 5 percent of the adult population in the United States. Beyond the U.S., the prevalence of asthma has considerably increased among other industrialized countries, by more than 30% over the last two decades. Given this, it follows that the impact of asthma on labor is a significant concern and it becomes important to assess asthma-related losses in productivity. The objective of this study was to determine the indirect costs due to asthma. METHODS: Retrospective analysis was conducted of the 1999 Medical Expenditure Panel Survey. The survey collected data from a nationally representative sample of 24,618 respondents and data from respondents' medical care and health insurance providers and employers. Data elements used in this study included medical conditions and employment information comprised of hourly earnings, hours worked, and disability days. Asthma patients who incurred disability days related to their condition were identified using International Classification of Diseases (ICD-9-CM) codes and variables denoting disability days. Indirect costs due to asthma-related absenteeism were calculated using the human capital approach. Sample estimates were weighted and projected to the population and 95 percent confidence limits for estimates were calculated using the Taylor expansion method. RESULTS: Indirect costs of asthma patients who missed workdays were $4,052,758,837 with mean indirect costs of $2,880 per patient (95% C.L. = $1,253 to $4,507). Relative to the entire population, mean indirect costs per person were $15. CONCLUSIONS: Productivity losses due to asthma were sizeable, with total indirect costs greater than $4 billion and mean indirect costs nearly $3000. With such work place consequences, additional studies of indirect costs resulting from asthma should be conducted in the U.S. and other countries to comprehensively quantify the costs of asthma and to further develop and implement responsive guidelines for diagnosis and treatment.