Severity of Type 1 diabetes mellitus (DM) at presentation was compared between south-east Sweden and Lithuania where incidence of childhood Type 1 diabetes is three times lower than in Sweden. New cases of diabetes at age 0-15 years from August 1995 to March 1999 in south-east Sweden and from August 1996 to August 2000 in Lithuania were included. Symptoms and clinical characteristics at diagnosis were recorded. Data about the close environment were collected using questionnaires. Lithuanian children were diagnosed in a more severe condition, mean pH 7.30 and HbA(1c) 11.5% compared with mean pH 7.36 and HbA(1c) 9.7% in Swedish children (P<0.0001). More Lithuanian than Swedish children were diagnosed in ketoacidosis (pH < or = 7.2, hyperglycaemia and ketonuria), 21.3 versus 7.3% (P<0.0001). Only 4.6% of Swedish children and 1.0% of Lithuanian children had no symptoms (P=0.007). Children in families with at least one first degree relative with diabetes (12.2% in Sweden and 8.4% in Lithuania, NS) had laboratory values at diagnosis closer to normal than sporadic cases in either country. Factors predicting ketoacidosis in Sweden were an unemployed mother and absence of infections in the 6 months before diagnosis. In Lithuania it was younger age and mother with less education. Additional educational activities for doctors are needed in countries with low incidence to reduce prevalence of ketoacidosis at onset.