To evaluate the possible role of the default mode network (DMN) in consciousness and assess the diagnostic or prognostic potential of DMN connectivity measures in the assessment of a patient group lacking cognitive awareness. DMN connectivity was established using independent component analysis of resting-state fMRI data in patients with reversible (n = 2) and irreversible (n = 11) coma following cardiac arrest and compared to healthy controls (n = 12). A present and intact DMN was observed in controls and those patients who subsequently regained consciousness, but was disrupted in all patients who failed to regain consciousness. The results suggest that the DMN is necessary but not sufficient to support consciousness. Clinically, DMN connectivity may serve as an indicator of the extent of cortical disruption and predict reversible impairments in consciousness.