The frequency of bacterial contamination of growth hormone solution and injection equipment use by 20 growth hormone deficient children was studied. In a cross-over study the children were randomized to begin using, for their growth hormone injections, either the recently developed growth hormone injection pen or the conventional syringe method. A comparison was then made of these two methods of injection over a 6-week period and the vials, pen-cartridges, syringes and needles were cultured. When the pen was used 5.3% of the 114 vials, 15.5% of the 110 cartridges and 11.2% of the 98 needles were found to be contaminated; when the syringe was used 3.5% of the 113 vials, 7.1% of the 98 syringes and 9.1% of the 99 needles were contaminated. To ensure microbiological safety during the preparation and injection of the growth hormone solution, regular instruction and reassessment of the injection technique for patients on long-term treatment are advocated. This applies even when disposable items are used. As no statistically significant difference was noted between the number of contaminated items used with either the pen or the syringe method, we conclude that the growth hormone pen is suitable for growth hormone administration.