Cardiac output monitoring is a common practice in critically ill patients. The PiCCO (pulse index continuous cardiac output) method requires artery cannulation. According to the manufacturer, the cannula in the radial artery should be removed after three days. However, longer monitoring is sometimes necessary. The aim of this study was to assess the incidence of radial artery occlusion (RAO) after three days of cannulation and to check whether five-day cannulation is related to a higher occlusion rate. An additional assessment was made to verify the presence of occlusion three, fourteen and thirty days after decannulation. The PiCCO cannula was inserted into the radial artery after the Barbeau test and Doppler assessment of blood flow. It was left for three or five days. Doppler was performed immediately after its removal and at three, fourteen and thirty days following decannulation. Thirty-seven patients were randomly assigned for three or five days of cannulation, and twenty-three of them were eligible for further analysis. RAO was found in thirteen (56.5%) patients. No statistical difference was found between the RAO rate for three and five day cannulations (p = 0.402). The incidence of RAO was lower when the right radial artery was cannulated (p = 0.022; OR 0.129). Radial artery cannulation with a PiCCO catheter poses a risk of RAO. However, the incidence of prolonged cannulation appeared to not increase the risk of artery occlusion. ClinicalTrials.gov ID NCT02695407.