Thrombolysis increases case fatality but reduces the proportion of disabled survivors in recent trials in acute ischaemic stroke, although some trials show much higher mortality rates than others. One possible explanation for the different outcomes between trials is that the treatment effect with thrombolysis varies with baseline prognostic factors such as stroke severity. We examined the interaction between baseline risk and thrombolysis on outcome using individual patient data from the Multicentre Acute Stroke Trial-Italy (MAST-I). A multiple logistic regression of the MAST-I data was performed to identify which factors, identifiable at randomisation, most strongly predict a poor functional outcome. We then stratified the patients into those with severe strokes and those with mild strokes and examined the effect of thrombolysis on (a) case fatality and (b) dependency at 6 months after the stroke in the 157 patients who received streptokinase alone and the 156 controls. Streptokinase was found to cause an absolute increase of about 3% in case fatality in both "severe" and "mild" strokes; however, there was a 12% reduction in the number of dead or dependent "mild" strokes but a 6% increase in "severe" strokes. The number of patients was small, and therefore neither finding was statistically significant. In this exploratory analysis, the hazard with streptokinase appears similar in "severe" and "mild" strokes, but the benefit may be greater in "mild" strokes. Thrombolysis may be more effective in patients with "mild" strokes, but more information is required to confirm this hypothesis.