Patients with inborn errors of metabolism (IEMs) have become an emerging and challenging group in the adult healthcare system whose needs should be known in order to implement appropriate policies and to adapt adult clinical departments. We aimed to analyze the clinical characteristics of adult patients with IEMs who attend the most important Spanish hospitals caring for these conditions. A cohort study was conducted in 500 patients, categorized by metabolic subtype according to pathophysiological classification. The most prevalent group of IEMs was amino acid disorders, with 108 (21.6%) patients diagnosed with phenylketonuria. Lysosomal storage disorders were the second group, in which 32 (6.4%) and 25 (5%) patients had Fabry disease and Gaucher disease respectively. The great clinical heterogeneity, the significant delay in diagnosis after symptom onset, the existence of some degree of physical dependence in a great number of patients, the need for a multidisciplinary and coordinated approach, and the lack of specific drug treatment are common features in this group of conditions.