A computer-aided method for the determination of colour fusion time (CFT) was developed. CFT indicates the acuity of the perception of monochromatic contours. CFT was determined in 36 patients with Parkinson's disease (PD) and compared with a group of 36 age- and sex-matched controls. Patients with PD generally had a shortened fusion time, especially for dark-green, light-blue and dark-red stimuli. The results give evidence to the hypothesis of a colour perception disorder in PD. The physiological and pathoanatomical basis of this phenomenon is unknown, but a functional deficit of cortical neurons may be a probable cause.