This paper analyses, on the basis of a series of general practice studies, the under-diagnosis of asthma and chronic obstructive pulmonary disease (COPD) in terms of the magnitude of the problem, and the implications and factors that contribute to adequate diagnosis. Most patients with chronic or persistent respiratory signs/symptoms present to the general practitioner (GP) and it is the GP who is usually responsible for diagnosis and treatment. An inherent problem of 'early' diagnosis of asthma and COPD is that signs and symptoms that patients experience must be followed over time to establish their chronic-recurrent nature. This approach fits well--in itself--with the general practice principle of continuity of care. The analysis was mainly based on the "Diagnosis, Intervention and Monitoring of COPD and Asthma (DIM-CA)" study that investigated the hypothesis that early intervention would enhance the effectiveness of (inhaled corticosteroid) intervention, and for this reason required detection of patients in as early a stage of their asthma/COPD as possible. Despite the fact that asthma and COPD are being diagnosed more frequently, the proportion of diagnosed to undiagnosed cases has remained stable over the years, pointing to an increase in prevalence in the population. A major factor in under-diagnosis is the fact that patients experience symptoms, but do not present these symptoms to a physician. Reluctance to present symptoms appears to be related to reluctance to take on the role of patient and take (inhaled) medication. This points to patient-perceptions and--values as an important factor in under-diagnosis. This finding is all the more relevant in the light of increasing indications of the value of early treatment. For GPs and primary care these findings are important. They imply that early intervention will only be possible when a case-finding approach, that evaluates pro-active outcome of respiratory signs/symptoms, is combined with an analysis of patients' perceptions and values.