BACKGROUND: Horror fusionis is an uncommon anomaly and is rarely reported in the literature. METHODS: Five adults with long-standing diplopia associated with horror fusionis were examined. RESULTS: All patients had strabismus since early childhood and had been treated at that time either with surgery, occlusion, and/or orthoptics/vision therapy. Prisms could not eliminate the diplopia. Graded occlusion was attempted with one patient but was not tolerated. Another patient with an asymmetrical dissociated vertical deviation could ignore the second image by fixating with the eye with the smaller deviation. Two patients reported the diplopia becoming less noticeable over the years. CONCLUSIONS: Because of its poor prognosis, the diplopia associated with horror fusionis must be differentiated from other types of diplopia occurring in adults with childhood onset strabismus.