The study aimed to 1) examine the point prevalence of asynchronous breathing in chronic fatigue syndrome (CFS) patients; 2) examine whether CFS patients with an asynchronous breathing pattern present with diminished lung function in comparison with CFS patients with a synchronous breathing pattern; and 3) examine whether one session of breathing retraining in CFS patients with an asynchronous breathing pattern is able to improve lung function. Twenty patients fulfilling the diagnostic criteria for CFS were recruited for participation in a pilot controlled clinical trial with repeated measures. Patients presenting with an asynchronous breathing pattern were given 20-30 minutes of breathing retraining. Patients presenting with a synchronous breathing pattern entered the control group and received no intervention. Of the 20 enrolled patients with CFS, 15 presented with a synchronous breathing pattern and the remaining 5 patients (25%) with an asynchronous breathing pattern. Baseline comparison revealed no group differences in demographic features, symptom severity, respiratory muscle strength, or pulmonary function testing data (spirometry). In comparison to no treatment, the session of breathing retraining resulted in an acute (immediately postintervention) decrease in respiratory rate (p < 0.001) and an increase in tidal volume (p < 0.001). No other respiratory variables responded to the session of breathing retraining. In conclusion, the present study provides preliminary evidence supportive of an asynchronous breathing pattern in a subgroup of CFS patients, and breathing retraining might be useful for improving tidal volume and respiratory rate in CFS patients presenting with an asynchronous breathing motion.