Today most adults with phenylketonuria (PKU) have followed a protein restricted diet from early infancy. This makes their disorder an invisible impairment which becomes visible only when food is served. When adhering to medical advice in public, adults with PKU find themselves in a liminal space, betwixt and between being medically ill and socially healthy. The affected adults may pass as being ‘normal’ if they ignore medical advice and eat ordinary food. By doing this, they risk severe neurological health consequences. Due to the rarity of this illness and limited knowledge regarding the health consequences of dietary intervention in adulthood, implications of adult PKU can be difficult to explain to others. Conscious stigma-handling strategies are therefore required. This article, which is based on qualitative interviews, discusses stigma and management strategies in the lives of eleven adults aged 20–30 with PKU.