OBJECTIVE: To determine the benefits of early photocoagulation in patients with type I versus type II diabetes. DESIGN: One eye of each of 3,711 patients was randomly assigned to early photocoagulation; the other was assigned to deferral of photocoagulation, with follow-up visits scheduled every 4 months and photocoagulation to be carried out promptly if high-risk proliferative retinopathy developed. Patients were categorized by age and type of diabetes. MAIN OUTCOME MEASURES: Best corrected visual acuity was measured at each study visit scheduled at 4-month intervals. Stereoscopic fundus photographs were taken and evaluated at baseline, 4 months, and yearly thereafter. Retinopathy severity was assessed from fundus photographs. Severe visual loss was defined as visual acuity of worse than 5/200 for at least two consecutive study visits. RESULTS: Previously published results of the Early Treatment Diabetic Retinopathy Study (ETDRS) demonstrated a statistically significant benefit of early photocoagulation in preventing severe vision loss. Further analyses demonstrate that this benefit of early photocoagulation is greater in patients with type II diabetes than in those with type I. The relative benefit of early photocoagulation in patients with type II diabetes is also seen for other outcomes (development of high-risk proliferative retinopathy, development of the combined end point [severe visual loss or vitrectomy], development of moderate visual loss, or development of legal blindness). The patients most likely to benefit from early photocoagulation had severe nonproliferative retinopathy or early proliferative retinopathy. Analyses from the Diabetic Retinopathy Study confirm the relative benefit of scatter photocoagulation for type II patients. Because of the high correlation between age and type of diabetes, analyses sub-grouped by age show similar results. CONCLUSION: These analyses suggest that patients with type II diabetes, or older patients with diabetes, are more likely to benefit from early scatter photocoagulation than patients with type I diabetes. The current standard of care is to initiate scatter photocoagulation as the severity of retinopathy approaches or reaches the high-risk stage, Provided careful follow-up is possible, ETDRS data do not show that initiating scatter photocoagulation prior to the development of high-risk proliferative diabetic retinopathy in patients with type I diabetes will reduce the risk of severe visual loss. ETDRS analyses do indicate that for patients with type II diabetes, it is especially important to consider scatter photocoagulation at the time of the development of severe nonproliferative or early proliferative retinopathy.