The paper describes a study involving 53 Swedish native multilingual students the objective of which was to investigate the relationship between their mother tongue oral and written proficiency as they themselves perceive it and their academic performance at two educational levels, i.e. secondary and tertiary. The study compares the students’ grade averages with the data obtained through questionnaires targeting their language proficiency and mother tongue use. The results show that while there is a positive correlation between the students’ degree of perceived proficiency (both written and oral) in the language originally spoken with their mother and their academic performance at their secondary school, this correlation seems to disappear once they enroll in their university studies. The paper discusses two of the possible reasons for this phenomenon on the background of threshold hypothesis and transitional perspective. According to these, native multilinguals benefit from their multilingual condition academically only when they reach a certain level of proficiency in the language(s) they use and only until their parents complete their linguistic assimilation in the country they have immigrated into.