Academic language proficiency, particularly academic writing, appears to be significantly related to academic achievement. The aim of the present study is to explore the relationship between written language proficiency and academic achievement, and to investigate the effects of a remedial academic writing programme. Academic writing was assessed by means of a Diagnostic Writing Test (DWT), administered to 5810 first year undergraduates in a Dutch university. Academic achievement was operationalised in terms of grade point average (GPA) and study credits obtained. One out of five students failed the DWT, even those who had obtained a (largely) sufficient exam mark for Dutch at high school. An additional analysis was conducted among over 300 students, in order to investigate the role of the DWT, as well as of other possible variables which may influence academic achievement (e.g. former training and time investment). Both the DWT and former training appeared to be related to GPA, but not the number of credits. Time-investment was associated with the number of credits obtained and GPA. Remediation appeared to be successful: about 75% of the participants who followed the course were able to convert a 'fail' on the DWT into a 'pass'.