Background Little is known about the prognosis of occupational asthma induced by high molecular weight proteins. Objective Our objective was to measure the clinical, immunological and employment outcomes of individuals with occupational asthma induced by detergent enzymes. Methods We undertook a workforce-based follow-up study in 35 (78%) of the 45 ex-employees from a single factory with occupational asthma. In each case the diagnosis was supported by evidence of specific sensitization and characteristic changes in peak flow or a positive response to specific bronchial provocation testing. Results This group had left the factory on average 37 months before study. On review 25 (71%) reported chest symptoms during the last month. Compared with when working at the factory, most (86%) reported that their symptoms had improved. Twenty continued to attend their general practitioner for respiratory symptoms and 19 still used asthma medications. Since leaving the factory 16 (46%) and four (11%) had found full-time or part-time employment, respectively; of these 16 found they were paid less than when they worked at the factory. The remaining 15 subjects had not had any paid employment. All but two had positive skin prick tests to one or more three detergent enzymes. The estimated half-life of serum-specific IgE antibodies was 20 months for protease, and 21 months for cellulase and amylase. Conclusions Population-based follow-up studies of the prognosis of occupational asthma are rare but probably avoid the bias in clinic-derived surveys. This study demonstrates that 3 years after the avoidance of exposure with detergent enzymes most patients continue to be troubled by, albeit improved, symptoms and experience difficulty in re-employment.