Forty-nine patients with a recent hemiplegia were studied for evidence of thrombo-embolism using 125I-labelled fibrinogen. Nearly half had evidence of thrombo-embolism and over one-third had a thrombosis in the paralysed leg. The decay of the plasma radioactivity correlated well with leg scanning or post-mortem evidence of pulmonary embolism, and can therefore be used as an independent indicator of thrombosis. The method has distinct advantages over leg scanning in geriatric patients since it can give earlier warning of thrombosis, allow monitoring for 2–3 weeks and give useful information in situations where leg scanning can be misleading.