Objectives.The authors compare the extent and nature of triazolam use before and after its deletion from a health maintenance organization's (HMO) open and advisory-type drug formulary.Methods.Benzodiazepine dispensings of HMO members were collected for the 3 years before (1989-1991) and for the 2 years after (1992-1993) the deletion of triazolam from the HMO's drug formulary. The number of triazolam users, their sex and ages, total annual exposure, estimated daily doses, and total days of annual exposure were calculated and compared before and after its deletion from the formulary.Results.From being the most frequently dispensed short-acting benzodiazepine in the HMO in 1989, the prescribing and use of triazolam decreased tenfold by the end of 1993. The effect on who was using the drug and how it was being used was less dramatic. The elderly were frequent longterm users and had the longest exposure to the drug.Conclusions.An open and advisory-type formulary can affect providers' drug selection behavior.