The adder (Vipera verus) is the only naturally occurring poisonous snake in Sweden. During one year, 136 patients were hospitalized due to adder bites in a population of 5.3 mill. (63% of the total population). These patients could be traced through the central register on diagnosis codes and have been studied retrospectively. The main purpose of the investigation was to study the epidemiology and to estimate the degree of poisoning, thereby getting an idea of the frequency of really severe cases. Minor local symptoms occurred in 27% of the patients, whereas 46% suffered mild, 15% moderate and 12% severe poisonings. The average duration of hospitalization was 1.6, 3.8, 5.5 and 7.6 days, respectively, for these four groups. Shock, CNS disturbances, anaphylactic reactions, extensive edemas, renal dysfunction and severe anemia were typical findings in the group with severe reactions. Although no deaths occurred, the proportion of severe cases contrasts with the current conception that severe envenomation after adder bites is rare. The mortality due to adder bite has been low during the last decades. Earlier the situation was different, and during 1911-78 altogether 44 deaths due to adder bite were registered in Sweden.