BACKGROUND Asthma with incomplete reversibility of airway obstruction (IRAO) may often be associated to smoking-induced changes. Nevertheless, a high proportion of patients showing IRAO have never smoked. These patients with IRAO often share features of patients with chronic obstructive pulmonary disease (COPD). Although IRAO is still a poorly defined condition, it has been associated with a higher morbidity and mortality than asthma with complete reversibility of airway obstruction (CRAO) or even COPD alone. A high prevalence of comorbidities could contribute to the reported poorer clinical outcome in IRAO, in comparison to CRAO or COPD alone. AIM To determine the prevalence of past and current comorbidities in IRAO patients compared to patients with CRAO or COPD. METHODS This was a retrospective, cross-sectional study. Demographic data, clinical characteristics and 36 predetermined comorbidities documented from self-report and chart review, were recorded from smoking-associated IRAO (S-IRAO), non-smoking IRAO (NS-IRAO), CRAO and COPD patients. RESULTS A total of 199 patients were included in the final analysis (111F/88M, mean (±SD) age of 63 ± 10 years). The CRAO group had more comorbidities than the three other groups, but this difference was significant only with the NS-IRAO group (P = 0.04). For most comorbidities, the prevalence of comorbidities in both IRAO sub-groups was intermediate between CRAO and COPD, with significant differences between S-IRAO and NS-IRAO only for hypertension (P = 0.03), nasal polyps (P = 0.002) and pneumonia (P = 0.04). Typical asthma-associated comorbidities tended to be more prevalent in NS-IRAO patients and COPD-associated comorbidities in S-IRAO patients. CONCLUSION In this study, the prevalence of comorbidities was not superior in patients with IRAO, compared to those with CRAO or COPD alone. The prevalence of comorbidities in the two main types of IRAO patients reflects exposure to cigarette smoke and asthma-related mechanisms.