In a retrospective study, all cases of anaphylactic shock (AS) occurring in a hospital catchment area during a 13-year period were analysed. Twenty cases were found, giving an incidence of 3.2 cases per 100,000 inhabitants per year, and one patient died. The precipitating agents were penicillin (7 cases), aspirin (3), food (2), and bee or wasp sting (8). The drugs had all been given orally. Penicillin-related AS was much commoner than its incidence reported by the Danish national health authorities. In three cases, previous anaphylactic reactions had occurred to the agent in question. We concluded that AS rarely occurs outside hospital, but that oral penicillin is a more important cause than was previously thought. Many recurring episodes of AS should be preventable, since hypersensitivity tests for both penicillin and hymenoptera venoms are available, and hyposensitization with respect to the latter is feasible.