A number of epidemiological investigations have identified asthma prevalence in cleaners around 1.5–2.0 times those of reference populations. There are around 700,000 cleaners in the UK, asthma prevalence is around 8%, and that suggests a high burden of work-related disease. However, a clinical diagnosis of occupational asthma in cleaners is established relatively rarely. We have investigated the hypothesis that this discrepancy occurs because cleaner’s asthma is a form of low dose irritant asthma that is visible to epidemiologists but does not have the typical clinical features of occupational asthma. A questionnaire was sent to 1400 cleaners working in local hospitals and universities. 14% had a previous diagnosis of asthma, and in 32% of these the asthma started after they began work as a cleaner. Investigations for possible occupational asthma comprised paired measurements of airway responsiveness at and away from work (n = 13), serial PEF analysed using OASYS-2 (n = 13) and a structured clinical history (n = 10). 5 subjects had a greater than 3 fold improvement in PD20 away from work, and 2 subjects had OASYS score >2.5 indicating a probable occupational effect. 1 subject had both. The clinical histories were sent to 9 physicians with an interest in occupational asthma who were asked to score them for the likelihood of occupational asthma on a scale 0 to100% with and without the OASYS scores and the airway responsiveness measurements. Before seeing the investigation results, 7 of the 90 individual scores (9 physicians x 10 subjects) were above 50% indicating that the diagnosis of occupational asthma was thought likely. After seeing the investigation results, 29 of the 90 scores were above 50%. The mean probability score based on the history alone did not exceed 50% for any cleaner but was above 50% for 2 cleaners when the investigations were taken into account. These findings support the view that cleaner’s asthma has features that make it difficult to identify from the clinical history.