Clinical and laboratory data on 20 cases of subacute sclerosing panencephalitis (SSPE) diagnosed in Finland in the years 1960–1969 are reported. The patients had measles at an early age. Of the 14 patients whose age on contracting measles is known, nine had it before two years of age. The academic performance of six out of the eleven children attending school was exceedingly poor several years before the onset of typical symptoms of SSPE. This may be an early sign of the disease. All sera of the eleven patients with SSPE studied had moderate or high titers of both complement-fixing (CF) and platelet-aggregating (PA) antibodies to measles virus. In the 45 “normal” postmeasles sera serving as controls, this correlation between the CF and PA tests was practically nonexistent. Despite positive CF reactions in 43 of the 45 control sera, only six were positive in the PA test, and even in these the titers were low. In the gel-precipitation tests with measles-virus antigen, sera from patients with SSPE gave three to six strong precipitation lines and the cerebrospinal fluids, one to two lines. About 50% of the controls who had had measles developed precipitating antibodies in their sera; the highest number of lines was two.