Congenital hypoaldosteronism due to an isolated aldosterone biosynthesis defect is rare. We report a 4 month old female infant who presented with failure to thrive, persistent hyponatremia and hyperkalemia. Investigations revealed normal serum 17 hydroxy progesterone and cortisol. A decreased serum aldosterone and serum 18 hydroxy corticosterone levels with a low 18 hydroxy corticosterone: aldosterone ratio was suggestive of corticosterone methyl oxidase type I deficiency. She was started on fludrocortisone replacement therapy with a subsequent normalization of electrolytes. Further molecular analysis is needed to ascertain the precise nature of the mutation.