To describe the Larsen index and its strengths and weaknesses. I reviewed the original publication, a number of published modifications, tests of reproducibility, and some clinical studies that have used the index. The Larsen index is essentially a grading of erosion severity that has satisfactory reproducibility for use in groups of patients. The Larsen index can clearly distinguish different rates of erosive progression in groups of patients with different characteristics or treatments. Recent studies using the Larsen index have identified clear treatment effects. Difficulty in measuring the success of previous treatments designed to hold back the progression of erosions relates to the inefficacy of those treatments, not insensitivity of the Larsen index.