We have analyzed the impact of taking an associated lab course on the final exam scores in two large introductory physics courses. Performance between students who did and did not take the lab course was compared using final exam questions from the associated courses that related to concepts from the lab courses. The population of students who took the lab in each case was somewhat different from those who did not enroll in the lab course in terms of background and major. Those differences were taken into account by normalizing their performance on the labrelated questions with scores on the exam questions that did not involve material covered in the lab. The average benefit on lab-related questions for students who took the lab, in both courses, was within 0.6% of the score of students who did not, with an uncertainty of less than 2%. This result raises questions as to the effectiveness of labs at supporting mastery of physics content. � 2015 Author(s).