With the virtual explosion of biomedical information, the medical educator regularly faces a quandary regarding how much to include in the medical curriculum. Opinions differ regarding how much of the available information on a particular topic should be presented in a medical school lecture. To understand better the effect of lecture information density on learning by medical students, one of the authors gave a basic clinical lecture only or clinical lectures with varying amounts of semirelated information. Tests which measured only retention of the basic material were given before lecture attendance, immediately after the lecture, and 15 days later. The results indicated that increasing the information density of a lecture reduced retention of the basic information. The memory loss apparently was due to information presented late in the lecture displacing facts learned by the students earlier in the same hour. The data suggest that lectures to medical students are more effective aids to learning when the information density is limited to a few main points that are "essential to know."