Smith-Magenis syndrome is characterized by multiple congenital anomalies and mental retardation caused by the heterozygous deletion of chromosomal region 17p11.2. We present a long-term follow-up study of a girl with Smith-Magenis syndrome and West syndrome. West syndrome became apparent at 7 months of age. Since then, mental retardation, particularly in terms of language development, became increasingly more obvious. The patient's spasms and hypsarrhythmia disappeared after a course of adrenocorticotropic hormone therapy, but focal seizures reappeared at the age of 3 years and 3 months. Her craniofacial dysmorphia and mental retardation became increasingly evident compared to her condition at the onset of West syndrome. Chromosome analysis detected the characteristic 17p deletion, which was then confirmed via fluorescent in situ hybridization analysis. This is the second report of a patient with Smith-Magenis syndrome and West syndrome; taken together, these results suggest that Smith-Magenis syndrome may be a further cause of West syndrome.