ADHD is one of the widespread neurological disorders among children. While a substantial amount of research have addressed the issues related to assessment practices and diagnosis criteria among majority language speaking children, ADHD among bilingual children or linguistic minority children has not yet been addressed and discussed so much in the research circles. The percentage of bilingual children with immigrant background in main stream schools in many countries is quite high. Despite this global demographic tendency, underdiagnostisation and assessment of bilingual children with inattention, over activity and impulsivity are being considered to be a psychiatric, psychological and educational challenge. In this paper we address several critical aspects of the assessment practices and medical diagnosis of bilingual children with immigrant background based on a research project. The paper presents also some solutions as an alternative to one-sided intelligence-test based approaches. We stress the importance of multidimensional, multisource and bilingual assessment model for identifying the knowledge-related and language-related elements of the academic learning gap that these children usually experience prior to and during the assessment period.