In a study spanning several years and including more than 1600 students, lab attendance was strongly correlated with lab grades (r = 0.64), lecture attendance (r = 0.49), and course grades (r = 0.60) in an introductory biology course. Regardless of the semester, lab attendance was lowest during the first week of the semester, and students who missed the first lab of the semester were significantly more likely to miss more labs and earn lower grades than other students. Students who missed labs, and especially those who missed more than one lab, earned disproportionately lower grades in lab and in the course than did students who attended all labs. These results indicate that (a) lab attendance is a strong predictor of grades in lecture and lab, (b) students most likely to attend lab are also most likely to attend lecture, and (c) first-week absenteeism can be used to identify students disproportionately likely to earn low grades, and (d) instructors can use lab attendance to quickly, easily, and accurately identify students at-risk for low grades. (Contains 2 figures and 1 table.)