The purpose of this paper was to examine how clients on the unit for adults at Södemalms district administration (Stockholm, Sweden) with severe mental illness and substance abuse differs from other clients with substance abuse at the baseline and at follow-up one year later. We have used a quantitative method and analyzed material from ASI-interviews performed at the adult drug abuse unit at Södermalm during the time period 2002-2010. At the baseline the clients with substance abuse and co-occurring severe mental illness have more problems than other clients, primarily in the area of mental health. They spend more time alone and are more likely to engage in illegal activities. Interviewers assess their situation as more serious and that they need more help in all of ASI's areas of life than other clients. Persons with severe mental illness have improved during the follow-up. However, the numbers of homeless people have increased and fewer have jobs. Meanwhile, more people have a regular income from the social security, their somatic status has improved, fewer hang out with people who have an active addiction, they engaged not so much in crime. Furthermore, the proportion of heavy use alcohol and narcotics has declined. Mental health improved significantly. The proportion meeting the threshold of severe mental disorder is reduced from 47% to 29%. The risk of suicide attempts in the last month reduced from 5% to 1%.