Background and purpose: Hyperdense middle cerebral artery sign (HMCAS) on CT is a well known indication of thromboembolic arterial occlusion. Its disappearance after thrombolytic therapy is poorly described. Taking the rate of HMCAS disappearance as a surrogate for MCA recanalisation, its prognostic value after intravenous thrombolysis was examined. Methods: 1905 stroke patients with HMCAS on admission CT scan in the Safe Implementation of Treatment in Stroke-International Stroke Thrombolysis Register (SITS-ISTR) were studied. On follow-up CT scans 22-36 h after thrombolysis, HMCAS disappeared in 831 cases, persisted in 788 and was uncertain in 122; followup CT was not done in 164 cases. Results: Patients whose HMCAS disappeared were younger (median age 67 years vs 69 years for persistent; p = 0.03), with milder stroke (admission National Institute of Health Stroke Scale (NIHSS) score was 16 vs 17; p<0.005) and were less likely to have early infarct signs on admission CT (26% vs 33%; p<0.005). Patients with disappearing HMCAS were more likely to have early improvement in NIHSS score (median improvement 2 vs 0 at 2 h; 4 vs 1 at 24 h), be independent at 3 months (42% vs 19%), with fewer deaths (15% vs 30%) than those with persistent HMCAS. In multivariate analysis, HMCAS disappearance independently predicted functional independence and survival. Early NIHSS improvement independently predicted HMCAS disappearance. Conclusions: HMCAS disappeared after intravenous thrombolysis in about half of cases and these patients had twice as good outcomes compared with those with persistent HMCAS. The prognosis in patients with MCA occlusion that persists after intravenous thrombolysis is poor, which may indicate the need for an alternative treatment approach to this subgroup.