In 6 type II diabetic patients, which showed no satisfactory blood sugar profiles under oral sulfonylurea monotherapy, a placebo controlled cross over pilot trial was performed to evaluate the effect of additive transdermal insulin application. In comparison to the monotherapy with sulfonylurea, the combination with transdermal insulin showed in 4 of the 6 patients a significant reduction of the daily mean blood sugar values as well as a slight increase of insulin serum concentrations. The 2 patients, who had no glucose reductions also showed a slight increase of insulin levels during the combination phase. Relevant adverse reactions were not seen. The results suggest, that in suited patients an improvement of sulfonylurea therapy by combination with transdermal insulin is possible.