This study describes a classification of overall retinopathy severity based on the presence and severity of lesions graded in the Airlie House classification of diabetic retinopathy, and demonstrates its use in assessing progression of retinopathy over a 6-year period in a group of insulin-taking patients. One hundred and ninety-one insulin-taking patients with diabetes of at least 5 years' duration were identified from their doctors' records in 1970-1971. Patients were seen again in 1972-1973 and 1976-1977. Stereoscopic color fundus photographs of 7 fields in each eye were taken at each visit. All photos were graded using a modification of the Airlie House classification, which specified 6 levels of retinopathy for a given eye; when both eyes are considered an 11-step grading scheme results. Of all patients seen at the 2-year visit whose retinopathy was at risk of progressing, 41.2% showed progression of one level or more on the scale, 19.2% progression of two or more levels. At the 6-year examination comparable rates were 75.0% and 58.4%. Of patients whose retinopathy was level 4 in each eye (nonproliferative retinopathy of "moderate" severity) at the baseline visit, 13/21 (72%) had progressed to proliferative retinopathy in at least one eye at the 6-year examination. The classification scheme proposed appears useful for characterizing overall retinopathy severity of patients on the basis of gradings of fundus photographs. The data presented may be of help in planning trials of treatment aimed at slowing the development or progression of retinopathy.