OBJECTIVE--To study the effect of intensive therapy of diabetes on the progression to clinical albuminuria in insulin dependent diabetic patients with microalbuminuria. DESIGN--Randomised controlled clinical trial of intensive versus conventional therapy of diabetes for a median of 5 years (range 2-8). SETTING--Nine hospital based specialist diabetes centres in England and Wales. SUBJECTS--70 European insulin dependent diabetic patients aged 17-59 years with microalbuminuria (albumin excretion 30-199 micrograms/min), but without arterial hypertension, recruited from the nine hospital based specialist diabetes centres. INTERVENTIONS--Intensive diabetic therapy was allocated to 36 patients (27 men, 9 women) and conventional diabetic therapy to 34 (24 men, 10 women). MAIN OUTCOME MEASURES--Development of clinical albuminuria, defined as albumin excretion greater than 200 micrograms/min on at least two consecutive occasions, and rate of change of albumin excretion. RESULTS--Mean glycated haemoglobin concentration, similar at baseline in the two groups (intensive therapy group 10.3% (SEM 1.9%), conventional therapy group 9.8% (1.6%)), fell significantly (by 14%) in the intensive therapy group only. A significant glycaemic separation between the two groups was maintained for up to three years. Progression to clinical albuminuria occurred in six patients in each group. Blood pressure, similar at baseline, fell significantly by 1 mm Hg (95% confidence interval -4.20 to 1.43) per year in the conventional therapy group, but the difference in the rate of blood pressure change between the groups was not significant. Independent of treatment assignment, a mean blood pressure above the group mean (93.6 mm Hg), but not the glycated haemoglobin concentration, predicted progression to clinical albuminuria (relative risk 4.2, 95% confidence interval 1.3 to 13.0). CONCLUSIONS--Intensive therapy with improved glycaemic control for three years had no impact on the progression of albuminuria in insulin dependent diabetic patients with microalbuminuria. The reduction in blood pressure in the conventional therapy group may have affected outcome--in that arterial blood pressure rather than glycated haemoglobin concentration seemed to be the main predictor of progression from microalbuminuria to clinical albuminuria.