The occurrence of the tapeworm (Diphyllobothrium latum) in eastern Finland in 1975-81 was studied and findings were compared with previous results. The fish tapeworm has previously been shown to be a common health problem, especially in these parts of Finland. The prevalence rates of fish tapeworm eggs in large consecutive routine series of hospital laboratories have decreased to a level of 1-4% among the population. The results also indicate some possible differences in the rates and trends between different provinces in eastern Finland. The reasons for the occurrence and changing trends of the fish tapeworm in eastern Finland are discussed.