Metamizole is a common analgesic and antipyretic drug. However, its use has been associated with a risk of side-effects involving agranulocytosis and aplastic anemia. These reactions are rare and unpredictable. The aim of this prospective study was to evaluate the risks of these complications in Poland, where this medication is available without prescription.Six hematological centers, serving approximately 40% (ca. 15 million people) of the country's population, participated in the study. All cases of agranulocytosis and aplastic anemia were recorded and their severity and association with the use of drugs, especially metamizole sodium, were evaluated. All the patients receiving cytostatic or immunosuppressive drugs were excluded.During the 12 months of the study, 2 cases of aplastic anemia and no cases of agranulocytosis were confirmed which may have been associated with the use of metamizole. Overall, we recorded 16 cases of agranulocytosis and 27 cases of aplastic anemia. The two cases of metamizole sodium-induced aplastic anemia were non-fatal in character.Considering the consumption of 112,300,094 tablets of metamizole in Poland a year, the figures obtained result in an incidence of 0.25 cases of aplastic anemia per 1 million persons per day of treatment; this value being less than that for other nonsteroidal anti-inflammatory drugs. Nevertheless, caution is recommended in the application of metamizole, particularly over long periods and in large doses.