For 5 years after surgery we measured visual acuity in 56 eyes that had undergone pars plana vitrectomy for proliferative diabetic retinopathy. A total of 44.7% eyes maintained improved visual outcome, 19.6% had unchanged visual acuity, and 35.7% were considered failures, as compared to preoperative measurements. Visual results obtained from each patient's visual acuity measured at 3 months, 6 months, 1 year, and 3 years after surgery were statistically compared with that obtained from the visual acuity at 5 years using the chi 2 test. Results showed that visual results recorded at 3 months and 6 months were significantly different from that recorded at the 5-year follow-up (p < 0.02, respectively). We found a tendency for the visual acuity to change during the initial 6 months after surgery and to stabilize by 1 year. Further analysis showed that the initial instability was due to the presence of preoperative retinal detachment. Postoperative visual functions also worsened with complications such as postoperative retinal detachment, glaucoma, and cataract formation.