Strictly speaking, CPEO is defined as paralysis of the external ocular muscles combined with ptosis. In Kearns-Sayre syndrome additional neurological or muscular deficits can be observed, sometimes even decades after the onset of ocular symptoms.Three patients with classical CPEO and one patient with Kearns-Sayre syndrome where included in the present study. The clinical examinations included electromyography, various serological parameters and histological biopsies.The onset of ocular symptoms with divergent strabism and ptosis could be dated back to the age of 7, 27, 38 and 44 years starting on one eye and progressing slowly over years to both eyes. In the Kearns-Sayre syndrome patient a general decrease in cardio-respiratory fitness and occasional collapses were noted. In three patients ptosis and strabism surgery led to the relief of the ocular symptoms.In CPEO respectively Kearns-Sayre syndrome the onset of the disease is variable. The final correct diagnosis is often delayed due to the mild beginning of the symptoms and the slow progression of the paralyses. Early EMG and muscle biopsy examination may facilitate the diagnosis.