We report a 10 year old girl with psychomotor retardation, myoclonic syndrome and extreme photosensitivity. Clinical symptomatology and EEG-findings were not compatible with any of the known myoclonic syndromes. The patient's remarkable phenotype with short stature, dystrophy, facial dysmorphia characterized by antimongoloid palpebral fissures, broad root of the nose, coarse nose, inner epicanthic folds, dysplasia of the external ears, higharched palate, syndactylism between 2nd and 3rd toes on both sides, small narrow hands is suggested of a chromosomal disorder. A ring-shaped chromosome of the G-group (21--22) could be found. After using the Giemsa- and C-banding technique this chromosome could be identified as number 21. Patients with ring chromosome 21 or 22 are phenotypically not distinguishable. This is due to duplication-deficiency-variability of ring chromosomes in growing somatic tissues. The cellular genotype of ring chromosomes varies between monosomy, trisomy and polysomie.