Objective: To determine the reliability of identifying patients diagnosed with asthma in general practice and their asthma exacerbation episodes from prescribing data. Data source: Automated database from 17 general practitioners (29,805 patients) in the northern Netherlands. Study design: Sensitivity, specificity and predictive values of four criteria for identifying patients diagnosed with asthma and two criteria for identifying asthma exacerbation episodes were calculated using the registered diagnosis as gold standard. Results: Prescription of one or more anti-asthma medications identified 95% of patients with an asthma diagnosis (positive predictive value 0.70), while two or more anti-asthma medications identified 71% (positive predictive value 0.79). A combination of oral corticosteroids or antibiotics identified 55% of exacerbations. Conclusions: Asthma patients can be identified reliably from prescribing data, but identification of asthma exacerbations was poor. The preference for one criterion over another for identifying patients diagnosed with asthma will depend on the reason for patient selection.