Patients with a central venous catheter (CVC) who receive intensive chemotherapy or a stem cell transplantation for haematological disease are at risk for developing CVC-related thrombosis. To study the incidence of thrombosis, 105 consecutive patients underwent serial Doppler-ultrasound and we evaluated whether clinically manifest thrombosis could be predicted by screening with Doppler-ultrasound. Patients with subclavian or jugular inserted CVCs were clinically assessed each day for signs and symptoms of thrombosis. Additional Doppler-ultrasound screens were performed weekly by an independent physician in all patients until CVC removal. Doppler-ultrasound recordings were assessed by two blinded observers. In cases of clinically suspected thrombosis, the attending physicians followed routine diagnostic and therapeutic procedures. The overall cumulative incidence of CVC-related thrombosis was 28.6% (30 of 105 patients). Of the 30 patients with thrombosis, 26 had subclinical thrombosis by Doppler-ultrasound, nine of whom developed clinically manifest thrombosis later. Four patients had clinically manifest thrombosis without prior abnormal Doppler-ultrasound. In cases of subclinical thrombosis the risk of developing symptomatic disease increased sevenfold (34.6% vs. 5.1%). Doppler-ultrasound screening may be useful to identify those patients that are at high and low risk for clinically manifest CVC-related thrombosis.