Problems of estimating college enrollments are addressed, with attention to shortfalls caused by smaller cohorts of traditional students and attempts to enroll adult students. Emphasis is placed on student credit hours (SCH) generated and on participation rates of adults. Full-time equivalency is defined as 24 SCH per student. Adult enrollment is defined as students 25 years and over in all categories of degree-credit instruction. Predictions based on a state population base are discussed, along with calculations at individual institutions. Data for a state are presented that indicate that an effort to increase the participation rates of adults will be necessary to increase the extent to which shortfalls can be moderated. Increasing the number of SCH per adult would also help in this effort but it may not be possible to increase it over 10 credits a year per student. Increasing participation rates will be an effective approach. The shortfalls in regular students have been gradual over the past few years, but beginning in 1983 a greater decline will be apparent. Another jolt will occur in 1990 when another large decline in freshmen will be apparent. (SW)