Inhaled medication is commonly prescribed for the treatment of asthma and chronic obstructive pulmonary disease (COPD), but is often not properly used by patients. A total of 316 patients suffering from asthma or COPD took part in a study that evaluated how patients utilized their metered-dose inhaler (MDI) or dry powder inhaler, using a standardized inhaler checklist. Two hundred eighty-one patients (88.9%) made at least one mistake in the inhalation technique. The mistakes were classified into skill and nonskill mistakes. Two hundred patients made one or more skill mistakes and 81 patients only made one or more nonskill mistakes. The most common skill error was “not continuing to inhale slowly after activation of the canister” (69.6%). The nonskill item most patients had difficulties with was “exhale before the inhalation” (65.8%). Patients who used an MDI made significantly fewer nonskill mistakes than patients using a dry powder device. Older patients had more difficulty with the correct use of the inhaler than younger patients. There was no difference in errors between men and women. In this patient sample, most patients failed to use their inhaler correctly. Regular instructions and checkups of inhalation technique are the responsibility of the physician and should be a standard and routine procedure.