BACKGROUND: Late onset asthma is associated with more severe disease and higher morbidity than in younger asthma patients. This may in part relate to under recognition of asthma in older adults, but evidence on the impact of patient age on diagnostic assessment of asthma in a specialist setting is sparse. AIM: To examine the impact of patient age on the type and proportion of diagnostic tests performed in patients undergoing specialist assessment for asthma. METHODS: Data from a clinical population consisting of all patients consecutively referred over a 12 months period to a specialist clinic for assessment of asthma were analysed. RESULTS: A total of 224 patients with asthma or suspected asthma were referred during the 12 month period; 86 adults aged <35 years, 95 aged 35-55 years and 43 aged >55 years. Symptom characteristics were similar, but adults >35 years had a lower lung function than younger adults, and were more frequently smokers. However, a regression analysis showed that older age was associated with a lower likelihood of diagnostic assessment with a reversibility test, a bronchial challenge test, or measurement of exhaled NO, independently of a known diagnosis of asthma, smoking habits and lung function at referral. CONCLUSION: A lower level of diagnostic assessment was observed already after the age of 35 years, indicating a risk for under diagnosis of asthma at an earlier patient age than previously thought.