Progressive retinal atrophy was studied in 17 Tibetan Terriers. The diagnosis was made on the basis of clinical signs of the disease, retinal histopathologic findings, or both. Affected dogs were the progeny of matings of affected or ophthalmoscopically normal dogs. Results of the mating supported a simple autosomal recessive mode of inheritance. The disease initially could be diagnosed by findings of night blindness and ophthalmoscopic signs of tapetal hyperreflectivity in affected dogs that were approximately 1 year old. Electroretinograms recorded from affected dogs, compared with those of clinically normal dogs of the same age, did not reveal appreciable abnormalities until affected dogs were 10 months old, at which time a reduction in the amplitude of the b wave was seen in response to a Ganzfeld white-light stimulus. The peak times of the response were unaffected. With progression of the disease, the electroretinographic b-wave amplitude was gradually reduced, and the electroretinographic response was extinguished in affected dogs by the time they were 30 months old. Early in the disease, rod and cone functions were affected equally, with more rapid loss of rod function developing only later in the disease. Fluorescein angiography of affected dogs did not reveal abnormalities earlier than could be detected by ophthalmoscopy. Despite the electroretinographic findings, histopathologic findings included patchy disorientation and disorganization of the outer segments of rods and cones in affected dogs as young as 9 weeks. With progression of the disease, rods were lost at a faster rate than cones, and atrophy of the inner retinal layer was observed.