Better knowledge is needed about the effectiveness of preparatory English language courses for the health professions. This study evaluated the scientific writing skills of students finishing their preparatory year of a bachelor of dentistry programme in Saudi Arabia. A cross-sectional study was conducted in 2014–15 among first-year dental students at the University of Dammam. Students were asked to write a 150-word English language assignment that was analysed for writing statistics and problems using Microsoft Word and plagiarism detection software. Of the 89 respondents, female students used a significantly greater number of words than did male students and their assignments had significantly lower Flesch reading ease scores. Male students had significantly lower odds of using references (OR 0.04) and higher odds of making punctuation and grammar mistakes (OR 2.63 and 3.91 respectively). One course of scientific writing in the preparatory year may not be enough to develop adequate writing skills among undergraduate dental students.