The objective of this study was to verify the incidence of phlebitis in a clinical medical unit. A prospective cohort study was conducted using nonparticipatory observations guided by a structured questionnaire, with 100 patients admitted to the clinical medical unit, in whom 234 peripheral venous accesses were used. Phlebitis was identified in 60% of patients and in 55.6% of the peripheral venous accesses, and most of them remained intact for more than 72 hours (53%). The presence of phlebitis per patient was associated with longer hospitalization on the clinical unit (p=0.002) and a greater number of accesses per patient (p<0.001). Length of retention of venous access for more than 72 hours was also associated with the presence of phlebitis (p<0.001). Phlebitis occurred in the majority of the patients and peripheral venous accesses analyzed. Therefore, it is necessary to intensify the training protocols for nursing staff in order to reduce the rates of phlebitis.