The authors report an 11-year-old girl with epilepsy, poor school-performance and minor behavioural disorders. The epilepsy is characterized by complex partial seizures, sometimes progressing secondarily into generalized tonic-clonic seizures, and is poorly controlled by medical treatment. Chromosomal analysis revealed a ring chromosome 20 mosaicism. The affected patient shows signs of ring chromosome 20 syndrome, characterized in the present case by poor school-performance, behavioural disorders and epilepsy.