Summer break sets students in grades K-12 back by at least one month of instruction and has the strongest impact on mathematics retention. This study investigates whether there is evidence of the summer gap effect at the university level for engineering students enrolled in the introductory calculus course sequence, and possible demographic mediators. Five years of final course grades for students who took the summer off between the first two courses in the Introductory Calculus for Engineers course sequence were compared with grades of students who took both courses in the same academic year. An analysis of covariance and an analysis of difference scores were used to examine the effect of summer break on grades in Calculus 2 and to control for time-invariant characteristics, including aptitude. Results show that university students who take the summer off lose the equivalent of about half a course grade more than do students who take the two courses in the same academic year. The summer break has the most impact on the strongest students, suggesting that it may contribute to the loss of many talented engineering majors. Gender does not mediate the summer gap effect, although international students outperform students who are US citizens.