Although the need for insight in factors influencing the quality of life of patients with an inborn error of metabolism is recognized, psychological adjustment of adults with metabolic diseases has not been properly studied. Adult patients with PKU were demonstrated not to differ from healthy controls in terms of their course of life (CoL) and health-related quality of life (HRQoL). However, adults with galactosaemia had a lower HRQoL with significant lower scores on the domains of cognitive and social function. This study investigated the CoL and the social demographical outcomes in these young adults with galactosaemia, and compared them with the general population and with PKU patients. A total of 15 (88%) adult patients with classical galactosaemia participated in this study. Classical galactosaemia patients had a delayed social and psychosexual development compared to their peers from the general population and to PKU patients. Also, they were significantly less frequently married or living together and significantly less frequently employed than the general population. Our study shows a stark contrast between patients with galactosaemia and patients with PKU, although both are diagnosed in the neonatal period and need life-long dietary restrictions. The observed difference is likely due to the long-term somatic complications frequently seen in galactosaemia and thus not due to the burden of a chronic disease necessitating life-long dietary restrictions. We conclude that it is essential that parents and clinicians encourage children with galactosaemia to participate in peer-related activities in order to stimulate social performance, which may result in a more normal CoL.