The present study looked at the effect of a helmet on cognitive performance under demanding conditions, so that small effects would become more detectible. Nineteen participants underwent 30min of continuous visual vigilance, tracking, and auditory vigilance (VTT+AVT), while seated in a warm environment (27.2 (±0.6)°C, humidity 41 (±1)%, and 0.5 (±0.1)ms-1 wind speed). The participants wore a helmet in one session and no helmet in the other, in random order. Comfort and temperature perception were measured at the end of each session. Helmet-wearing was associated with reduced comfort (p=0.001) and increased temperature perception (p<0.001), compared to not wearing a helmet. Just one out of nine cognitive parameters showed a significant effect of helmet-wearing (p=.032), disappearing in a post-hoc comparison. These results resolve previous disparate studies to suggest that, although helmets can be uncomfortable, any effect of wearing a helmet on cognitive performance is at worst marginal. © 2013 Elsevier Ltd and The Ergonomics Society.