AIM: To analyse the formation and disappearance rates of individual microaneurysms in mild background retinopathy. METHODS: Three fluorescein angiograms were taken at 1 year intervals during a 2 year follow up from 24 type 1 diabetics with mild background retinopathy. Microaneurysms were identified and localised twice from each angiogram using a computerised system for retrieval of the coordinates for each microaneurysm. Microaneurysms identified similarly in both sessions were then processed further to obtain rates of microaneurysm formation and disappearance, and microaneurysm count changes. RESULTS: In the whole material the total number of microaneurysms increased from 298 to 436 from baseline to 2 years. During the 2 year follow up 395 new microaneurysms formed and 258 microaneurysms disappeared. Of the microaneurysms present at baseline 174 (58%) were still present at the 1 year and 142 (48%) at the 2 year follow up. In patients with good glucose control (HbA1c < or = 7.5 mmol) microaneurysm formation showed a trend of being decreased whereas microaneurysm disappearance did not correlate with glucose control. CONCLUSION: Background diabetic retinopathy is a dynamic process. A significant proportion of microaneurysms present disappear within 2 years. This is compensated for by formation of new microaneurysms, the resultant net changes in microaneurysm counts being small. Microaneurysm formation and disappearance rates are new variables of diabetic retinopathy and may prove to be more sensitive indicators of the progression patterns of background diabetic retinopathy than microaneurysm count changes.