This study is a 7.5 year follow‐up of a population‐based series of twins with multiple sclerosis (MS) whose mean age now exceeds 50 years. The twin pairs were identified through the Cabadian nationwide system of MS clinics and were drawn from a population of 5,463 patients. After 7.5 years, the monozygotic concordance rate increased from 25.9 to 30.8 an the dizygotic‐like sex concordance rate from 2.4 to 4.7%. These results are very similar to those of other population‐based studies and to our own modified replication twin date reported here. We interpret the data to mean that MS susceptibility is genetically influenced, and a single dominant or even a single recessive gene is unlikely to account for this effect. The difference in concordance rates suggests that at least two or more genes are operative. These date also have importatnt implications for the nature of the environmental effect(s) in MS susceptibility. Most monozygotic twins are discordant even after a correction for age and magnetic resonance imaging findings. This unabiguously demonstrates the powerful effect of nonberitable factors.