During the 10-year period 1976-1985, a total of 154 cases of blood dyscrasia were reported in Sweden which were evaluated as having a probable or possible causal relationship with trimethoprim-sulphamethoxazole (T-SM). There were 61 cases of leucopenia (of which 16 had agranulocytosis), 28 cases of thrombocytopenia, and two of non-haemolytic anaemia. There were also 32 cases of bicytopenia and 31 cases of tricytopenia. The median age varied from 38 years in the leucopenia group to 81 years in those with tricytopenia. The overall fatality rate was 17%, ranging from 2% in the group with mild leucopenia to 52% in the group with tricytopenia. In relation to sales and prescription data, the overall incidence of reported T-SM blood dyscrasias was 5.3 per million defined daily doses, and among out-patients the incidence was one case per 18,000 prescriptions. Thus the overall incidence of any blood reaction to T-SM appears to be low. In relation to prescription data, elderly people were overrepresented among the serious reactions.