We conducted a cohort study to investigate the frequency of serious blood dyscrasias in patients age 10-74 years, taking antiepileptic drugs between January 1, 1990, and October 31, 1994. Main outcome measures were validated diagnoses of neutropenia, agranulocytosis, hemolytic anemia, thrombocytopenia, bicytopenia, pancytopenia, or aplastic anemia. A total of 29,357 recipients of antiepileptic therapy received 684,706 prescriptions. Among them there were 21 cases of serious blood dyscrasia of which only 18 could be considered to have a temporal relationship to drug use. Seventeen cases occurred in current users of carbamazepine, phenobarbital, phenytoin or valproate, and 7 in patients taking two or more drugs. Twenty of the 21 patients recovered. The overall rate of blood dyscrasias was 3-4/100,000 prescriptions. The rate in those age less than 60 years was 2.0 (range 0.9-3.6)/100,000 prescriptions compared with 4.0 (range 1.6-8.2) for those age 60 or older. The overall rate of neutropenia was 1.2 (0.5-2.3)/100,000 prescriptions, compared with 0.9 (0.3-1.9) for thrombocytopenia and 0.4 (0.1-1.3) for hemolytic anemia. Rates did not differ among the four drugs. Serious blood dyscrasias are rare in patients taking antiepileptic agents.