OBJECTIVES: This study aimed to project future costs of asthma and chronic obstructive pulmonary disease (COPD) in the Netherlands for the period 2000 - 2025. METHODS: Gender-, age- and (for COPD) disease-stage specific data on direct, medical costs per patient were obtained from a cost of illness study for the year 2000. Cost projections over the period 2000 - 2025 were based on two types of projections of the prevalence of asthma and COPD. Both prevalence projections were combined with two types of cost projections; first assuming constant costs per patient and second adding asthma and COPD-specific trends in costs for hospital care (asthma, -4.7% and COPD, -4,1% per year) and medication (+4,9% per year) and general trends in remaining cost categories. RESULTS: In 2000 the annual costs per asthma patient were €315, compared to €915 for a COPD patient. Assuming constant costs per patient, costs were projected to increase from €141 million in 2000 to amounts between 167 and €181 million for asthma and from 280 to €443 - 495 million for COPD. Severity stage specific COPD costs increased from 22.6 to 51.2 for mild COPD, from 104 to 148 for moderate, from 99.0 to 140 for severe and from 54.5 to €156 million for very severe COPD. Including trends costs categories resulted in estimates between 460 and 497 for asthma and 1023 and €1130 million for COPD. CONCLUSION: Projections of future costs show that the absolute and relative increase in costs over the period 2000 - 2025 is higher for COPD than for asthma.