Background: Asthma and chronic obstructive pulmonary disease (COPD) have major symptoms in common. However, the mode of the underlying chronic airway inflammation is different. There is still no single diagnostic test that can be considered a gold standard to distinguish asthma from COPD. Objectives: To determine the diagnostic accuracy for asthma and COPD of a series of diagnostic steps in a population older than 40 years with probable obstructive airway disease (OAD) in primary care. Methods: In this prospective cohort study, patients without a certain diagnosis underwent a work-up, including office spirometry by their general practitioner (GP). They were then referred to a pulmonologist, and they had control visits with their GP. The diagnostic gain of subsequent steps was calculated for 2 endpoints, namely the specialist's opinion and the GP's final opinion. Results: Up to 60% of the patients failed to consult with the pulmonologist. For this subgroup, the office spirometry induced significantly more diagnostic congruency than any other diagnostic step. The specialists rejected 44.5% of the diagnoses made by the GPs, including spirometry. High values of diagnostic gain were found after the office spirometry and after the specialist's advice. Up to 25% of the population taking bronchodilators were judged not to suffer from OAD. Conclusions: Office spirometry added significantly more to the diagnostic certainty of the GPs than questionnaires, history and clinical examination. A pulmonologist's advice contributed more to diagnostic certainty than any other diagnostic step. Nevertheless, 26% of the diagnoses made by the chest physicians were reconsidered by the GPs.