Intravenous infusion therapy has become an indispensible part of intensive care. Problems of bacterial contamination during this therapy are well known. To check on the possible routes of contamination we examined the infusion system in its several parts (infusion solution, infusion system, connection between infusion system and catheter and content of syringes). The highest rate of contamination was found at the connection between the infusion system and the catheter after use for 24 h (26.7%, 39 out of 146 probes); just at the beginning of the infusion we found bacterial growth in 7.1% (10 out of 141 probes). After injection of drugs into the system the infusion solution was contaminated in 1.9% (6 of 320 probes). The system for measuring the central venous pressure was contaminated in 2.7% (4 of 148 probes). At the end of infusion the infusion solutions were contaminated in 3.1% (9 of 287 probes). Different drugs in syringes in no case were contaminated. In most cases (59 probes) we found gram-positive bacteria (87.3%), in only seven cases (9.7%) gram-negative bacterias and in two cases Candida tropicalis. Our results show that the extrinsic or in use contamination plays the most important part in bacterial contamination of the infusion system. Infection control of intravenous therapy necessitates care in the hygienic standard adopted during the infusion and injection procedures.