One of the most frecuent complications of enteral nutrition is formula contamination. The objective of this study was to evaluate the frequency and routes by which enteral formula contamination occurs. Two types of formula were evaluated, one powdered and one liquid, with sterile re-used containers. The results show that the most important way of contamination is in the formula-preparing unit by manipulation or use of contaminated utensils. The utilization of re-used containers contributes to contamination in a lesser degree. The solution administration sets have a high degree of contamination, over 85% and this is not modified by the use of sterile material. Probably there is also a retrograde contamination mechanism.