Patients who need self-injection of insulin are educated beforehand to handle vials and syringes without bacterial contamination. However, not a few of them forget what they were told about the sterile technique during a long period of injection at home. Since 1988 a pen-typesyringe, which is considered to be more potent against bacterial contamination because of its mechanical structure, has become available in Japan. The aim of this study is to detect the percentage of contaminated vials in the patients' home and to compare traditional vials with pen-type vials in terms of potency against contamination. Two hundred eight vials were collected from 168 patients. Four traditional vials out of 163 (2.4 %) and 1 pen-type out of 45 (2.2%) were contaminated. Propionibacterium acnesgrew up from 2 vials, staphylococcus epidermidis from 2 vials and unidentified gram (+) rods from 1 vial. The patients using contaminated vials were from 30 to 65 in their age, enough skillful to handle syringes, good or poor in the control of DM and without disturbed vision. Thus, the contamination may have been derived from their technical deterioration after several years of injection at home. Therefore, re-education to keep their sterile technique should be given to them at appropriate intervals at the out-patient clinic.