Introduction There is conflicting evidence regarding impairment of sustained attention in early Alzheimer's disease (AD). We examine whether sustained attention is impaired and predicts deficits in other cognitive domains in early AD. Methods Fifty‐one patients with early AD (MMSE > 18) and 15 healthy elderly controls were recruited. The sustained attention to response task (SART) was used to assess sustained attention. A subset of 25 patients also performed tasks assessing general cognitive function (ADAS‐Cog), episodic memory (Logical memory scale, Paired Associates Learning), executive function (verbal fluency, grammatical reasoning) and working memory (digit and spatial span). Results AD patients were significantly impaired on the SART compared to healthy controls (total error β = 19.75, p = 0.027). SART errors significantly correlated with MMSE score (Spearman's rho = −0.338, p = 0.015) and significantly predicted deficits in ADAS‐Cog (β = 0.14, p = 0.004). Discussions Patients with early AD have significant deficits in sustained attention, as measured using the SART. This may impair performance on general cognitive testing, and therefore should be taken into account during clinical assessment, and everyday management of individuals with early AD. Copyright © 2016 John Wiley & Sons, Ltd.