Background: Differential diagnosis of asthma and VCD can be challenging. Previous research (e.g. Newman, Mason & Schmaling, 1995) has suggested a number of risk factors for VCD. Aims and Objectives: To compare co-morbidity characteristics of patients with complex breathlessness referred to a tertiary Airways service. To analyse whether specific factors are associated with an increased likelihood of confirming a diagnosis of VCD. Methods: Descriptive and correlational statistics were used to analyse patient characteristics and co-morbid factors in a sample of 155 patients, of whom 83 had confirmed VCD. A logistic regression analysis was used to identify significant predictor variables for VCD. Results: The majority of patients diagnosed with VCD were female (72%). A high proportion of these patients had reflux (49%), rhinitis (41%) and/or co-morbid asthma (33%). Partial correlations showed that Forced Vital Capacity percent (FVC%) within lung function tests was significantly correlated with a diagnosis of VCD (rs= 0.329, n=73, p=0.005). Age, gender, reflux, rhinitis and Forced Expiratory Volume 1 percent were not significantly correlated with VCD diagnosis. A logistic regression was performed to assess the impact of FVC% on diagnosis of VCD. The model was significant: χ 2 (1, N=73) =10.38, p=0.001. The model explained between 13.3% and 21.8% of the variance in VCD diagnosis, and correctly classified 83.6% of cases. Conclusions: A number of possible co-morbid factors were identified. Lung function, as described by FVC%, was found to be a significant predictor for VCD diagnosis in a sample of patients with complex breathlessness. This measure may be useful in the differential diagnosis of asthma and VCD.