Malonyl-CoA decarboxylase (EC 4.1.1.9) deficiency (McKusick 248360) with malonic aciduria was first reported by Brown et al (1984) in a patient with short stature, repeated episodes of vomiting and mild developmental delay. A second patient with malonic aciduria and malonyl-CoA decarboxylase deficiency was reported by Haan et al (1986). The latter patient had a more severe clinical presentation that included vomiting, seizures, hypoglycaemia, mild acidosis and developmental delay. We report a patient with hypotonia, mild developmental delay, seizures and cardiomyopathy with malonic aciduria and malonyl-CoA decarboxylase deficiency