Objective - To determine the prevalence of an incorrect inhalation technique and to examine its determinants among primary care patients with asthma or chronic obstructive pulmonary disease (COPD). Design - Cross-sectional study. Setting - 28 general practitioners in The Netherlands. Subjects - 558 asthma and COPD patients, aged 16-75 years. Main outcome measures - Inhalation technique was assessed using a standardised inhaler-specific checklist. Pulmonary function assessment and questionnaires were used to collect data about inhaler, patient and disease characteristics. Results - Overall, 24.2% of the patients made at least one essential mistake in their inhalation technique. The type of inhaler appeared to be the strongest independent determinant of an incorrect inhalation technique. Compared to patients using the Diskhaler, patients using the Rotahaler/Spinhaler, Turbuhaler, Metered Dose Inhaler (MDI) or Cyclohaler/Inhaler-Ingelheim were at significantly higher risk of making inhalation mistakes (odds ratios (OR) were 16.08, 13.17, 11.60 and 3.27, respectively). Other significant determinants of an incorrect inhalation technique were low emotional quality of life (OR = 1.73) and being treated in a group practice (OR = 2.26). Conclusions - An incorrect inhalation technique is common among pulmonary disease patients in primary care. Our study suggests that especially patients using the Rotahaler/Spinhaler, Turbuhaler or MDI, patients with emotional problems and patients in a group practice are at increased risk for an incorrect inhalation technique.