Although the Addiction Severity Index (ASI) is one of the most frequently used measures in alcohol and other drug research, it has rarely been used to assess clinical and reliable change. This study assessed clients' clinical and reliable change at The Salvation Army residential substance abuse treatment centers in Australia. A total of 296 clients completed ASI interviews on admission to treatment and 3 months after discharge from treatment. Clients demonstrated significant improvement on all seven ASI composites. The range of reliable change for each ASI composite varied from 30% to 70%. More than two-thirds of clients experienced clinically significant improvement for alcohol and drug problems. Psychiatric distress was clinically reduced in 44% of clients. This research indicates that residential substance abuse treatment can make important differences in client's lives at a clinical and functional level. However, the research highlights the challenge of effectively targeting psychiatric comorbidity within alcohol and other drug abuse populations.