Great spatial variation in the incidence of IDDM is found among countries around the Baltic Sea, a relative small area on the global scale. We present recent data on IDDM incidence from countries around the Baltic Sea, monthly variation and time trends in incidence from the early 1980s to the early 1990s. The change in IDDM incidence was calculated from logarithms of incidence using linear regression. The incidence was high in the countries to the north and west from the Baltic Sea, being the highest in Finland (35 per 100,000/year) followed by Sweden (26), Denmark (22) and Norway (21). In the countries on the eastern and southern coast of the Baltic Sea the incidence was markedly lower, in Estonia the incidence (10) was the highest within these countries, though slightly less than one third of that in Finland, while it was in Lithuania 7, Latvia 7 and Poland 6. There was an increasing trend in incidence of IDDM in Finland, Norway and Poland. In Sweden the incidence increased from 1978 to 1984, but since then the trend has been flat. In Estonia, Latvia and Lithuania, no significant change in incidence was seen. The reasons for large differences observed in the incidence of IDDM between countries around the Baltic Sea area are unknown, but a complex interaction between genetic and environmental risk factors that can vary in different ethnic, socio-economic and cultural settings play an important role in this variation.