In a retrospective study, 113 bites which occurred in Switzerland within a 16-year period by either of the two indigenous adders (Vipera berus and Vipera aspis) were analyzed. 13 patients showed no signs of envenomation. Out of the other 100, 62 patients had merely minor (local edema only) and 24 moderate envenomation (vomiting, diarrhea, cramps, hypotension), while 24 had severe envenomation (shock and angioneurotic edema of the tongue and lips). No fatalities were recorded. In residents of the endemic area the bites occurred accidentally in 86% of the cases, while tourists were bitten after manipulating the snake in 42%. Specific antivenin was given to 49 of 95 hospitalized patients. In cases of severe envenomation (n = 14) a reduction of the median duration of hospitalization from 10 days in patients without antivenin therapy (n = 4) to 5 days in patients with antivenin therapy (n = 10) was obtained. In cases of moderate (n = 21) or minor envenomation (n = 49), antivenin therapy did not influence the hospital stay of 4 and 2 days respectively. In the 11 patients with bites not followed by envenomation the antivenin treatment increased the duration of hospitalization from 1 day (in 6 patients without antivenin) to 2 days (in 5 patients with antivenin). Side effects of the antivenin treatment, such as urticaria, angioneurotic edema, respiratory distress, fever and lymphadenopathy were noted in 4 out of 49 patients.