In brain damage patients with unilateral spatial neglect (USN), the differential diagnosis between the presence and absence of a unilateral visual half-field deficit (VHFD) is hampered by the similarity of their phenomenology. The absence of stimuli detection in the contralateral visual field, indeed, can be due to the co-occurrence of USN and VHFD or the sole presence of the USN. The disentangling of the two conditions is required to devise more specific rehabilitation programmes. Daini et al. [2002. Exploring the syndrome of spatial unilateral neglect through an illusion of length. Experimental Brain Research, 144(2), 224-237.] reported a difference in performance for the two conditions when the tasks required the bisection of Brentano illusory stimuli. Only when USN and VHFD co-occurred, the leftward illusory effect was disrupted. Based on previous findings, in this cross-sectional study, we developed the Brentano Illusion Test (BRIT), a clinical tool that helps the identification of VHFD in USN patients. The BRIT is a simple behavioural test of lines bisection aimed at verifying the presence or absence of implicit processing in USN and thus helping the diagnosis of VHFD in USN patients; it also provides normative data for the line bisection task and the length effect.