The aim of this study is to describe the essential characteristics of a family affected by the newly-described proximal myotonic myopathy (PROMM) The clinical, laboratory and genetic findings are described and compared with those reported in the literature, and the clinical spectrum of the manifestations that are similar to but distinct from myotonic dystrophy (MD) is also explored. This has practical implications because the cases so far described suggest that the long-term prognosis with PROMM seems to be more favourable than that of patients with MD.