Objective: To determine the validity of work-related self-reported exacerbation of asthma using the findings from serial peak expiratory flow (PEF) measurements as the standard. Methods: Adults with asthma treated in a health maintenance organisation were asked to conduct serial spirometry testing at home and at work for 3 weeks. Self-reported respiratory symptoms and medication use were recorded in two ways: a daily log completed concurrently with the serial PEF testing and a telephone questionnaire administered after the PEF testing. Three researchers evaluated the serial PEF records and judged whether a work relationship was evident. Results: 95 of 382 (25%) working adults with asthma provided adequate serial PEF data, and 13 of 95 (14%) were judged to have workplace exacerbation of asthma (WEA) based on these data. Self-reported concurrent medication use was the most valid single operational definition, with a sensitivity of 62% and a specificity of 65%. Conclusions: a work-related pattern of self-reported asthma symptoms or medication use was usually not corroborated by serial PEF testing and failed to identify many people who had evidence of WEA based on the serial PEF measurements.