Clients in treatment and aftercare programmes with a history of long-term abuse of substances like alcohol, cannabis, methamphetamine or heroin often exhibit serious problems while coming back to the working process and managing complex responsibilities and difficulties. It is very important to find a job in the aftercare process for many reasons. It is a source of living, it enables clients to plan their own housing, and it makes repayment of debts possible. Job is also important for establishing new social relations which are not connected with problematic drug environment. Last but not least employment contributes to a reconstruction of a healthy rhythm of life and meaningful spending of time. Thus it is very important for decreasing a probability of a relapse. This process can be disrupted by the deteriorated cognitive functioning. These problems may not be evident in an everyday life routine while talking with friends or performing automatic activities, but can be evident in demanding working tasks, because the job usually requires concentration, memory, decision making etc. The Goal of this study is an evaluation of potential benefits an individual cognitive performance assessment can have in treatment and aftercare. The paper summarizes pilot research in which a complete Neuro-psychological battery of diagnostic methods such as Auditory Verbal Learning Test (AVLT), Verbal fluency Test (FAS), Trail Making Test (TMT), Rey-Osterrieth Complex Figure (ROCF), Benton Visual Retention Test (BVRT), Stroop Colour Word Test (SCWT) and Numeric square is used for assessment of cognitive functions of a small sample of treated clients (N=24). We present two case histories with detailed results. In some cases, in line with the expectations, the worsened cognitive functioning is evident. We recommend a cognitive training focused on attention and memory for three clients. In other cases, surprisingly, we can find normal cognitive functions quality, but another obstruction for optimal performance, such as enhanced anxiety, can be detected. Probably, long-term drug abusers can have a larger tendency to be anxious, due to a problematical self-image and fighting other difficulties. An effect of comorbidity should also be taken into account. A depression or anxious syndromes or other common problems can take their part anyway. But it is important to notify that in our sample are not present any confirmed psychiatric diagnoses other than the syndrome of dependence. Standard aftercare programmes devote a lot of attention to another aspect of a treatment like psychosocial, physical and/or socioeconomic issues, but the cognitive assessment and the cognitive training is not a standard part of their effort. We believe that individualized and detailed assessment of cognitive functions of clients in the aftercare programmes can contribute to creating an optimal therapeutic plan which has a larger chance to be effective and successful. This paper is a pilot study for a more extensive quantitative study with larger number of clients in which we will try to compare specific effect of different kind of drugs. We would like to describe the long-term effect of abuse of drugs like alcohol, cannabis, methamphetamine and heroin. Negative effects of drugs for human cognitive functioning are evident. But we can't say that an effect of specific drugs itself is the only cause. Patterns of use, combinations with other substances, and effects of additives or regime of life and so on are also very important. From the point of view of methodology, this area is therefore rather problematic. We would like to bring more awareness of this theme using already tested battery of psychodiagnostic methods.