Patients with a dysexecutive syndrome often have severe disabilities in daily life activities. The aims of this study were to use a naturalistic experimental task to assess patients' disabilities, and to study the nature of the cognitive disorders underlying them. Execution of a cooking task involving multi-tasking (Chevignard et al., 2000) was studied in 45 patients with a dysexecutive syndrome following acquired brain injury. Patients made significantly more errors and were slower than controls; more than half of the patients did not achieve the goal and demonstrated dangerous behaviours. Those results were significantly correlated to the results of the Six Elements Task and to a behavioural questionnaire. They were also correlated to brain injury severity and to patients' cooking habits. This naturalistic assessment is clinically relevant to better assess patients' dysexecutive impairments in complex activities of daily living. Correlations of the results in the cooking task with the neuropsychological assessment highlighted the role of the dysexecutive syndrome in patients' disabilities, indicating control alterations rather than planning disorders, difficulty in dealing with the environment, and inhibiting inappropriate actions. The role of attention and prospective memory was also underlined, whereas other cognitive functions did not influence task performance.