Insulin is a growth stimulating hormone for several malignant tumors. In experimental breast cancer, diabetes induction with alloxan, strep-tozotocin or diazoxide leads to remission and intensifies the effect of an ovariectomy and of antiestrogen treatment. Clinically, in one of nine breast cancer patients, a partial remission of dermal metastases of seven months duration was achieved following a tamoxifen-induced remission by additional treatment with 200 mg diazoxide daily. In two other breast cancer patients, diazoxide led to stable disease of four (monotherapy) and eight months duration. Increased insulin levels can enhance the effect of simultaneous cytostatic therapy in a number of experimental systems. Initial clinical trials of combined therapy with methotrexate (MTX)/5-fluorouracil (5FU) and glucose/insulin indicate an increased cytostatic effect of this combination in some patients with breast cancer and in two patients with colon cancer, who were both pretreated with the same MTX/5FU regimen without glucose/insulin. In a case treated with high dose methotrexate therapy, the decrease of serum MTX-levels was slightly accelerated during and markedly delayed after the end of the glucose/ insulin-infusion.