Acute transverse myelitis (ATM) may be a manifestation of multiple sclerosis (MS) and was reported to be more common among Japanese MS patients than in Caucasian MS patients. Recently there are arguments whether clinical manifestations of MS may have changed. Therefore, we studied the frequency of ATM in MS and the clinical subtypes of MS in 86 clinically definite MS patients whose onsets were in 1970-1979, 1980-1989, and 1990-1998 in Sendai City, Japan. Fifty-six of the patients were women and 30 were men. Forty-four patients had the conventional form of MS (C-MS) commonly seen in Western countries, and 42 had optic-spinal or spinal forms of MS (OSS-MS). Twenty MS patients had ATM, and all of them were belong to optic-spinal form of MS. ATM was not seen in any cases of C-MS. The mean onset age (years) of the clinical subtypes was 25.5 in C-MS, 34.1 in OSS-MS without ATM, and 30.9 in OSS-MS with ATM. Among the patients whose onset of the disease was in 1970-1979, 60.0% of them were cases of OSS-MS with ATM, but such cases were markedly decreased to 5.3% in 1990-1998. In contrast, the frequency of C-MS increased to 63.2% in 1990-1998 compared with 20.0% in 1970-1979. Analysis of the data by the year of birth of the patients showed similar results. Our data suggest that the frequency of ATM in MS markedly decreased, and that of C-MS increased during the last 30 years in Sendai, Japan. Since the genetic background of Japanese has not changed, some exogenous factors, such as food, infectious microorganisms, and chemicals in our environment, may be responsible for the change.