<jats:sec><jats:title>Background</jats:title><jats:p>Although the Head Positioning in acute Stroke Trial (HeadPoST) showed no effect of the flat head position (FP; vs sitting up head position (SUP)) on functional outcome, we hypothesised that it could still offer benefits if commenced early in those with acute ischaemic stroke (AIS) of at least moderate severity.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>Subgroup analysis of HeadPoST in participants with National Institutes of Health Stroke Scale (NIHSS) scores ≥7, ≥10 and ≥14, randomised to FP or SUP <4.5 hours of AIS onset on functional outcomes defined by a shift in scores on the modified Rankin scale (mRS) and death/disability (mRS scores 3–6), and any cardiovascular serious adverse event. Logistic regression analyses were undertaken adjusted for study design and baseline risk factors.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>There was no significant differential treatment effect in patient subgroups defined by increasing baseline NIHSS scores: adjusted OR and 95% CI for ordinal shift and binary (3–6) mRS scores: for NIHSS ≥7 (n=867) 0.92 (0.67 to 1.25) and 0.74 (0.52 to 1.04); NIHSS ≥ 10 (n=606) 0.80 (0.58 to 1.10) and 0.77 (0.49 to 1.19); NIHSS ≥14 (n=378) 0.82 (0.54 to 1.24) and 1.22 (0.69 to 2.14).</jats:p></jats:sec><jats:sec><jats:title>Conclusions</jats:title><jats:p>Early FP had no significant effect in patients with moderate–severe AIS.</jats:p></jats:sec><jats:sec><jats:title>Trial registration number</jats:title><jats:p><jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="clintrialgov" xlink:href="NCT02162017">NCT02162017</jats:ext-link>.</jats:p></jats:sec>
