Tight blood glucose control has been speculated to reduce late complications in insulin-dependent diabetics but results from individual studies have been inconsistent. We have done a meta-analysis of sixteen randomised trials of intensive therapy to estimate its impact on the progression of diabetic retinopathy and nephropathy and the risks of severe side-effects. In the intensive therapy group, the risk of retinopathy progression was insignificantly higher after 6-12 months of intensive control (odds ratio [OR] 2.11). After more than two years of intensive therapy the risk of retinopathy progression was lower (OR 0.49 [95% confidence interval 0.28-0.85], p = 0.011). The risk of nephropathy progression was also decreased significantly (OR 0.34 [0.20-0.58], p < 0.001). The incidence of severe hypoglycaemia increased by 9.1 episodes per 100 person-years (95% Cl -1.4 to +19.6) in the intensively treated patients. The incidence of diabetic ketoacidosis increased by 12.6 episodes per 100 person-years (95% Cl, 8.7-16.5) in the patients on continuous subcutaneous insulin infusion. Long-term intensive blood glucose control significantly reduces the risk of diabetic retinopathy and nephropathy progression but long-term continuous subcutaneous insulin infusion was associated with an increased incidence of diabetic ketoacidosis, and intensive therapy may cause more severe hypoglycaemic reactions.