Facioscapulohumeral muscular dystrophy (FSHD) is a common muscular disorder, but clinical and genetic complications make its diagnosis difficult. Southern blot analysis detects a smaller sized EcoRI fragment on chromosome 4q35 in most facioscapulohumeral muscular dystrophy patients, that contains integral number of 3.3-kb tandem repeats known as D4Z4. The problems for the genetic diagnosis are that southern blotting for facioscapulohumeral muscular dystrophy is quite laborious and time-consuming, and the D4Z4 number is only estimated from the size of the fragment. We developed a more simplified diagnostic method using a long polymerase chain reaction (PCR) amplification technique. Successful amplification was achieved in all facioscapulohumeral muscular dystrophy patients with an EcoRI fragment size ranging from 10 to 25 kb, and each patient had a specific polymerase chain reaction product which corresponded to the size calculated from the number of D4Z4. Using southern blot analysis, more than 90% of facioscapulohumeral muscular dystrophy patients have a smaller EcoRI fragment than 26kb in our series, and the number of D4Z4 repeats is precisely counted by this polymerase chain reaction method. We conclude that this long polymerase chain reaction method can be used as an accurate genetic screening technique for facioscapulohumeral muscular dystrophy patients.