Measurement of costs and outcomes: The primary outcome will be the "presence of definite or probable symptomatic or asymptomatic DVT in the veins behind the knee or in the thigh detected on either of two screening compression Doppler ultrasound scans (performed at about 7−10 days and 25 − 30 days as part of the research) or contrast venography or MRI(only if clinically indicated) within 30 days of randomisation". This is regarded as a clinically important outcome, acceptable to patients and staff and has been shown to be practical to collect in the vast majority of patients in the ongoing CLOTS Trials 1 & 2. Secondary outcomes measured in hospital and collected at hospital discharge by reviewing the casenotes will include: death within 30 days; Presence of definite or probable DVT in the popliteal or femoral veins detected on a screening Compression Doppler ultrasound scan which had not been suspected clinically before the scan,Definite (i.e. excluding probable DVTs) symptomatic or asymptomatic DVT in the popliteal or femoral veins detected on either a Compression Doppler ultrasound scan or contrast venography or MRI direct thrombus imaging within 30 days of randomisation; Any definite or probable symptomatic or asymptomatic DVT (i.e. including DVTs which only involve the calf veins); Confirmed fatal or non−fatal PE; Adherence to allocated treatment.