As awareness of dementia as a major public health issue grows, increasing numbers of individuals with subjective memory impairment (SMI) are presenting to memory services. Many with SMI are cognitively healthy and require only reassurance. In the absence of reliable and easily available biomarkers of dementia, clinical signs may be the most effective way of differentiating cognitively healthy SMI individuals from those with underlying brain disease. Collateral history is important in the assessment of memory complaints, so patients are routinely instructed to bring a relative, friend, or a carer to clinic with them. Attending the clinic alone despite these instructions, the “attended alone” sign, is shown in this study to be not only a robust marker of absence of dementia but also of cognitively healthy individuals with SMI.