Data from the 1970 National Longitudinal Survey of Labor Market Experience are employed to estimate the foregone earnings of college students. An estimation of earnings functions from male and female youths who are not enrolled in school, and the potential earnings of enrollees is derived by substituting the characteristics of enrollees into the earnings functions of the nonstudents. Actual earnings are subtracted from potential earnings to derive estimates of foregone earnings. The estimation procedure provides a range of estimates by employing alternative earnings functions and assumptions concerning potential hours per year. Self-selection bias, part-time versus full-time employment, and levels of enrollments are also considered. Results are compared to other studies, and a projection of earnings foregone is made for 1975 and 1980. The estimation methodology is described and illustrated with numerous equations. The results suggest that earnings foregone per student vary greatly between males and females, part-time and full-time students, and between levels of enrollments. It is estimated that earnings foregone by college students in 1980 range from $42.69 billion to $73.37 billion. (SW)