Twenty diabetic patients participated in a study to assess if multiple use of needles for insulin injection systems (Pens) is safe under normal daily conditions. The previous mean duration of Pen therapy was 16.3 months. During this time, the 20 patients carried out altogether more than 33 000 injections without any sign of local infection despite needle reuse. Patients were told to use needles if possible for 1, 3, 6, 9, and 12 injections before bacteriological assessment. Bacteriological investigation of these needles showed no contamination, except with one needle used three times, which was colonized with coagulase negative Staphylococcus. In contrast, half of the needles' plastic ground points which touched the skin were contaminated. No signs of infection were observed at the injection sites throughout the study. We conclude that, based on the bacteriostatic effects of commercially formulated insulin and on the siliconisation of needles' surfaces, bacterial growth is sufficiently prevented. Therefore, we can recommend the reuse of pen needles as a simple, safe and cost-beneficial procedure.