To determine how the rate and duration of naltrexone use have changed over time, we analyzed paid claims for patients of a large mid-Atlantic health insurer with more than three million members who were taking naltrexone among the subpopulation of 1.5 million members who were 18 years or older and had prescription drug and behavioral health benefits provided through the health plan. We calculated the number of filled naltrexone prescriptions per 10,000 members and the total number of days that naltrexone was supplied per patient from 2000 to 2002.