OBJECTIVES: To identify variables patients use to determine the severity of their asthma, the perceived severity (PS), using a fuzzy decision-making analysis (FDMA). To compare these variables with those involved in the assessment of asthma severity according to the global initiative for asthma (GINA) guidelines, the objective severity (OS). PATIENTS: Outpatients (51 men, 62 women), aged (m+/-SD) 42.9+/-16.3 years with (% patients) mild intermittent (6.2), mild persistent (15.9), moderate (65.5) and severe (12.4) asthma. DESIGN: Cross sectional, observational study. METHODS: Both OS (rated by doctors) and PS (rated by patients) were rated as mild intermittent, mild persistent, moderate, or severe. Variables involved in OS assessment, variables self-assessed by patients (dyspnea, perceived treatment efficacy, asthma-related quality of life questionnaire [AQLQ]), patients' sociodemographic characteristics, and asthma characteristics, were evaluated with questionnaires. These variables were pooled, and considered as potential variables patients might use to determine their PS. They were tested against the PS measurement using FDMA. This identified variables patients actually used to determine PS. RESULTS: On the day of consultation, 68.1% of patients classed their asthma as mild intermittent or mild persistent, 23.9% as moderate persistent, and 8.0% as severe persistent. There was a significant discrepancy (p<0.01) between PS and OS with a clear patient tendency to underestimate asthma severity as compared to OS. Patients determined PS level according to variables assessing their asthma perception, i.e., AQLQ measures and dyspnea, but not variables involved in OS assessment, such as symptom frequency or knowledge of their peak flow rates. Duration of asthma and treatment characteristics were also involved. CONCLUSION: FDMA identified variables patients used to determine PS. It highlighted a discrepancy between patients' and doctors' perceptions of asthma severity, suggesting that assessment of asthma severity should consider both patients' and doctors' perceptions of the disease and includes an AQLQ measure.