The incidence rates of multiple sclerosis (MS) in Denmark were estimated as a result of a continuous nationwide epidemiological survey since 1948 by the Danish Multiple Sclerosis Registry (DMSR). Among cases notified to the DMSR, 6,478 met the diagnostic criteria and had onset of MS from 1948 through 1982. The crude annual incidence rate was 4.45/100,000 and the lifetime cumulative incidence rate was 0.32%. The female/male ratio was 1.37:1. Crude incidence rates declined during the period 1952-1967. After 1967 the incidence seemed to rise again but in an irregular pattern. The decline in incidence involved only cases up to the age of 35. The subsequent increase was most marked in females and in the age groups over 45 years.