As transplantation has progressively become a more viable option for children with life-threatening illness, ensuring that adolescents do not lose their new organ secondary to medication non-adherence is paramount. The first step to addressing non-adherence is adequate assessment of this construct. In this investigation, we introduce the MACS. The MACS includes self-report and drug assay levels. Self-report is a subjective measure with a low false-positive rate, but is vulnerable to social desirability. Drug assays are an objective measure of drug ingestion, but values suggestive of non-adherence may be influenced by medical complications and timing. The MACS builds on the strengths of both methods and attempts to contain their weaknesses. The sample in this study consisted of 82 adolescent solid organ transplant recipients. The non-adherence rate using the MACS in this sample was 61%. Initial data to support this system are promising. The occurrence of rejection episodes and mortality were significantly related to membership in the Genuinely Non-adherent category. Beyond providing initial support for the MACS, we discuss the clinical implications of this adherence classification system.