Ring chromosome 20 (r(20)) syndrome is a rare disease characterized by refractory epilepsy, moderate mental retardation and particular electroencephalographic disorder with non-convulsive status epilepticus. Here, we report a new case of r(20) syndrome in a 12 year old female who presented minimal dysmorphism, generalised tonic-clonic and absence seizures refractory to medical therapy and behavioural troubles. Among 20 cytogenetically analysed cells, 14 (70%) exhibited a 46,XX,r(20)(p13q13.3) karyotype and 6 (30%) showed a normal 46,XX caryotype. Interphasic FISH using centromeric probe of chromosome 20 detects the presence of a chromosome 20 monosomy in 7% and a duplicated ring chromosome 20 in 8% of studied cells. Metaphase FISH using chromosome 20 telomeric probes and specific probes of CHRNA4 and KCNQ2 genes detects the absence of any deletion in the ring chromosome 20. Clinical symptoms of r(20) syndrome are attributed to telomeric partial monosomy generated by ring chromosome and causing an haploinsufficiency of two epilepsy genes CHRNA4 and KCNQ2. However, our patient presents the typical epilepsy disorder but no detectable deletion in the ring chromosome 20. We speculate that clinical features of ring chromosome 20 syndrome are caused by low mosaicism of chromosome 20 monosomy caused by the loss of the ring chromosome 20.