PRIMARY OBJECTIVES: Awake surgeries of slow-growing tumours invading the brain and guided by direct electrical stimulation induce major brain reorganizations accompanied with slight impairments post-operatively. In most cases, these deficits are so slight after a few days that they are often not detectable on classical neuropsychological evaluations. Consequently, this study investigated whether simple visuo-manual reaction time paradigms would sign some level of functional asymmetries between both hemispheres. Importantly, the visual stimulus was located in the saggital plane in order to limit attentional biases and to focus mainly on the inter-hemispheric asymmetry. METHODS AND PROCEDURES: Three patients (aged 41, 59 and 59 years) after resections in parietal regions and a control group (age = 44, SD = 6.9) were compared during simple uni- and bimanual reaction times (RTs). MAIN OUTCOMES AND RESULTS: Longer RTs were observed for the contralesional compared to the ipsilesional hand in the unimanual condition. This asymmetry was reversed for the bimanual condition despite longer RTs. CONCLUSION AND CLINICAL IMPLICATIONS: Reaction time paradigms are useful in these patients to monitor more precisely their functional deficits, especially their level of functional asymmetry, and to understand brain (re)organization following slow-growing lesions.