The concept of the minimal clinically important difference (MCID) is intended to provide a measure of relevance for a statistically applied in patients with COPD. Clinically important differences are those differences relevant to the individual patient and important to the patient's life. However, people's difference in a diagnostic parameter perception of what is important vary. Furthermore, physicians may rate the significance of a particular marker and its difference which can be achieved by a pharmacological intervention differently from the patient. Thus, the major problem with defining an MCID for any measure is that the most important differences, which require the most subtle measures for an individual patient, are likely to have the least general application. Conversely, measures that can be generalised are unlikely to have much individual importance and will be very crude tools for an individual assessment. In medical trials both, statistical rigor and clinical relevance are generally required, and MCID is without doubt a key application tool defining treatment success or treatment failure. This paper gives an update on the concept of a minimal important difference of most relevant parameters in COPD treatment.