Hypoglycemic agents were operationally classified into 4 groups: insulin #1all preparations), sulfonylureas (acetohexamide, chlorpropamide, glimepiride, glipizide, glyburide, tolazamide, and tolbutamide), metformin, and troglitazone. Acarbose, miglitol, repaglinide, and rosiglitazone were infrequently used during the study period, and their exposure data are not presented. Consecutive dispensings of drugs of the same group to the same person defined an exposure era, which started with the first dispensing and ended on the earliest of the following: (1) the date of the last dispensing plus the days-of-supply of the last dispensing plus 30 days, (2) termination of membership, or (3) June 30, 1999. During the study period, a patient could have 2 or more exposure eras to the same group of drugs or 2 or more groups of drugs with overlaps and intervening gaps. Total exposed person-time for insulin, sulfonylurea, metformin, or troglitazone was the summation of length of exposure eras to each drug group over all study patients. Patients with overlapping exposure eras to 2 or more groups of drugs contributed person-time to each group.