Traditionally, it has been recognized that patients with myotonic dystrophy type 1 (MD-1) - also known as Steinert disease -, they show a specific behaviour, not including those who suffer from mental or neurodevelopmental diseases. The neurological substrate of this behaviour is described. The aim of this text has two purposes. The first intention is that clinical staff, when faced with a patient with MD-1, always consider the cognitive aspects of the disease. On the other hand, it is intended to combat preconceived ideas about the particular behaviour of these patients., (Copyright © 2022 Sociedad Española de Médicos de Atención Primaria (SEMERGEN). Publicado por Elsevier España, S.L.U. All rights reserved.)