The costs of addiction treatment services are an important determinant of the cost-effectiveness of a program, and therefore, of relevance to addiction treatment providers, insurance companies and, patients. Several methods have been developed to estimate the costs of substance abuse treatment services. One such method is the drug abuse treatment cost analysis program (DATCAP), which collects resource use and cost data from the treatment program perspective and has been used in numerous published economic evaluation studies. However, no single widely-used, standardized instrument is currently available to estimate costs specifically incurred by clients in treatment. In response to that need, this article introduces the Client DATCAP and presents process, survey-specific, and quantitative findings from a Pilot Study to estimate the client costs of attending outpatient and inpatient treatment. The preliminary findings suggest that the self-administered Client DATCAP is a feasible and practical instrument for estimating costs incurred by clients in treatment, with completion time amounting to less than 10 min. Furthermore, client costs had a considerable range across respondents, with time costs consistently accounting for the largest cost component. Findings from the Pilot Study led to the development and release of edition 2 of the outpatient and inpatient modules of the Client DATCAP.