When diet therapy is no longer effective in keeping the fasting plasma glucose level < 6 mmol l-1, a basal insulin supplement from a long-acting insulin such as ultralente can be added instead of using a sulphonylurea or metformin. The dose of insulin required can be predicted from the level of the fasting plasma glucose and the degree of obesity, which provides an index of the accompanying insulin resistance. The risk of hypoglycaemia is minimal provided that the dose is adjusted according to the fasting plasma glucose concentration and the patient can continue a normal life-style without restrictions concerning exercise or the size of individual meals. If given in appropriate doses a basal insulin supplement does not induce marked weight gain and insulin therapy is equally appropriate in patients with insulin deficiency and insulin resistance. Maintaining near-normal glucose concentrations probably outweights a putative risk of hyperinsulinaemia. In more severely affected patients, such as those with sulphonylurea failure, soluble insulin to cover meals in addition to a basal insulin supplement is needed. At this stage it is usual to stop tablet therapy and treat patients with either a basal and prandial insulin regimen or with twice daily soluble and isophane mixtures. Nevertheless, in elderly patients in whom regular meals cannot be guaranteed, continuing with sulphonylurea therapy and adding a basal insulin supplement can be a safe and effective way of preventing hyperglycaemic symptoms.