The aim of this study was to determine the efficacy of a colorimetric pupil light reflex (PLR) device (Melan-100(®), U.S.A.) in dogs with sudden acquired retinal degeneration syndrome (SARDS; 16 cases), progressive retinal atrophy (PRA; 10 cases) and optic pathway disease (6 cases). The colorimetric device detected PLR abnormality in 32, 16 and 9 eyes with SARDS, PRA and optic pathway disease, respectively, whereas white light detected PLR abnormality in 18, 11 and 9 eyes with SARDS, PRA and optic pathway disease, respectively. SARDS dogs displayed miosis, while optic pathway disease dogs displayed mydriasis in a blue light examination. Thus, colorimetric PLR may be a useful method for determining whether electroretinography (ERG) or magnetic resonance imaging (MRI) should be performed for dogs with acute blindness.