STUDY OBJECTIVE: Catheter-related thrombosis (CRT) is a major complication of central venous catheters (CVCs). However, the incidence, onset, and dependence of CRT on CVC material and/or type in critically ill surgical patients is unknown. Therefore, we here investigated the incidence, onset, and dependence of CRT on a variety of risk factors, including CVC material and type, in critically ill surgical patients. DESIGN: Prospective, investigator-initiated, observational study. SETTING: A surgical intensive care unit at a university hospital. PATIENTS: All critically ill patients with CVCs (surgical: 79.8%/medical: 20.2%) who were treated in our surgical intensive care unit during a six-month period. INTERVENTIONS: None. MEASUREMENTS: All CVCs were examined for CRT every other day using ultrasound, starting within 24 h of placement. The primary outcome was the time of onset of CRT, depending on the type of CVC (three to five lumens, three different manufacturers). The CRT risk factors were analyzed using multiple Cox proportional hazards regression models. MAIN RESULTS: We included 94 first-time CVCs in the internal jugular vein. The median time to CRT varied from one to five days for different types of CVCs. Within one day, 37 to 64% of CVCs and within one week, 64 to 100% of CVCs developed a CRT. All but one of the CRT observed were asymptomatic and caused no complications. Multiple regression analyses of CRT risk factors showed that beside cancer and omitting prophylactic anticoagulation, some types of CVC were also associated with a higher risk of CRT. CONCLUSIONS: Almost all CVCs in the internal jugular vein in critically ill surgical patients developed an asymptomatic CRT in the first days after catheterization.