With the increase in the number of multilingual children in Germany the problems associated with the diagnosis of conspicuous language development in these children are becoming more urgent. This contribution addresses the importance, basic aspects and problems of diagnostics in the native language (L1) of multilingual children, using Russian as an example, and introduces the first version of a Russian language proficiency test for multilingual children (SRUK). It is known that if only the environmental language (L2) is tested, multilingual children can be misdiagnosed, because in the earlier stages the L2 development of bilingual children might show similarities to monolingual Specific Language Impairment (SLI). Thus, testing the L1 of multilingual children helps to more precisely determine SLI. One of the basic requirements for L1 testing is knowledge of the language, ideally at (near) native speaker level. Furthermore, with increasing age the acquisition of L1 proceeds parallel to attrition. Another important point is the amount of input and continuity of language use, factors which also impact L1 proficiency. Only with the accurate consideration of all these factors can the preciseness of a diagnosis in L1 be assured. The diagnostics in L1 with SRUK considers both the production and perception of lexicon, morphological marking, and basic grammatical constructions on the sentence level. Also, the input amount is matched with language proficiency.