Two female (aged 12 and 14 years) patients with horizontal gaze palsy and progressive scoliosis (HGPPS) from two different Turkish families were evaluated through ophthalmic, neurological, radiological, and orthopaedic examinations. Each proband had one affected sibling with HGPPS. Both patients had absence of horizontal eye movements and simultaneous adduction of both eyes on attempted horizontal gaze associated with miosis. One proband was observed to have this finding at age 9, which had not been present at 5 years old. Neuroradiological imaging revealed brainstem hypoplasia (split pons sign) and scoliosis. The two index patients harboured different homozygous mutations that were heterozygous in the parents, who are obligate carriers. The emergence of convergence and presence of miosis during lateral gaze suggests that simultaneous adduction occurs as a result of the convergence substitution phenomenon. Substituted convergence in cases with HGPPS may occur with different mechanisms than substituted convergence in cases with pontine lesions.