Platelet reactivity was studied immediately after and throughout the first 6 months following myocardial infarction. Its modification by sulphinpyrazone was observed. Out of 65 consecutive patients admitted to the coronary care unit, ten did not meet the protocol criteria. Fifty five received either placebo or sulphinpyrazone in a double-blind trial for 6 months, treatment being started within 6 days of infarction. Forty four patients completed the study (19 on placebo and 25 on sulphinpyrazone). An additional ten patients commenced treatment within one day of infarction and were studied daily for one week. Platelet hyper-reactivity was demonstrable in all patients at presentation and persisted throughout the 6 months in the control group. This effect was reversed by sulphinpyrazone, the modification of reactivity being established by the third day of treatment.