To determine the rate of phlebitis cases and their risk factors, the authors carried out a prospective follow-up study in an internal medicine unit. 553 catheters pertaining to 266 patients were analyzed. The length of time a catheter was used did not have any significant bearing on the development of phlebitis. The rate of phlebitis cases was more frequent when medication was infused at intermittent intervals. The use of intravenous antibiotics significantly increased the risk of phlebitis when compared to the administration of other types of medication.