To determine the relationship between vitreous fluorophotometry (VF) and severity of diabetic retinopathy (DR) 13 patients with mild to moderate background DR starting continuous subcutaneous insulin infusion were followed up serially for 12 months. They were studied by colour stereo retinal photographs, which were assessed by the Wisconsin Grading System, and by VF, which was assessed by the permeability index of the retina for fluorescein. By four months the severity of DR on colour photographs had deteriorated by at least one level in at least one eye in eight patients. In four patients the DR did not deteriorate. Assessment of anatomical abnormalities by severity of DR on colour photographs correlated well with the functional abnormalities of the blood-retinal barrier(s) assessed by VF, especially for the macular field. Comparison of permeability index data in the patients developing preproliferative or proliferative features of DR (group A) with the same data in patients who did not develop such changes (group B) indicated that group A patients had more severe DR than did group B patients at entry. Grading of colour photographs showed a similar trend but with greater overlap. Considerable overlap in fluorescein permeability remained between those subjects with no visible DR and those with microaneurysms with or without haemorrhages and small hard exudates.