Clinical trials designed to establish the effectiveness of a pharmacotherapy for the treatment of drug abuse typically call for the collection and analysis of three urine samples per week to detect changes in drug use patterns. Examination of over 16,500 urine samples collected from 225 subjects during a one year buprenorphine/methadone clinical trial indicates that analysis of one weekly urine sample from those collected on a three-times-per-week fixed schedule provides essentially the same outcome information as analysis of all three weekly urines. Further, the percent of opiate-positive samples is constant across weekday, indicating that a single urine, randomly selected from those collected each week, is a valid indicator of treatment performance.