The rise in serum IGF I concentration during continuous subcutaneous insulin infusion (CSII) may be a contributory factor in the deterioration of diabetic retinopathy that sometimes occurs during this treatment but the relation of serum levels to the severity of retinopathy has not been previously studied. In twelve non-obese insulin dependent diabetics (age range: 22-41 yrs) with mean +/- SD duration of diabetes: 14.8 +/- 4.7 yrs, serum IGF I concentration, HbA1 and retinopathy score were estimated prospectively over twelve months following the institution of CSII therapy. After four months of treatment, eight patients showed deterioration of retinopathy by at least one level of severity. Serum IGF I concentration rose from a mean +/- SEM of 155 +/- 17.7 micrograms/l at entry to 199 +/- 23.1 micrograms/l at four months and by twelve months had returned to near initial values 163 +/- 17.4 micrograms/l. There was however, no significant correlation between retinopathy score and serum IGF I level by analysis of variance for the whole group, or in the group of diabetics whose retinopathy deteriorated. The rise in IGF I concentration over the first four months and subsequent decline in IGF I values over the next eight months was inversely related to HbA1 concentration (r = -0.58; P less than 0.05). One patient with early ischaemic retinopathy on entry, experienced a marked rise in serum IGF I corresponding to a rapid tightening of glycaemic control. At four months she developed florid proliferative changes requiring panretinal laser therapy.(ABSTRACT TRUNCATED AT 250 WORDS)