The present invention relates to stable crystalline (6 S )-N(5)-formyl-5,6,7,8-tetrahydrofolic acid, commonly referred to as levofolinic acid, in essentially pure 6 S diastereomeric form, and to a process for its preparation, comprising the steps of: - preparing a first aqueous or hydroalcoholic solution, having a pH value higher than 4.5, of a soluble levofolinate salt - providing a batch of water, or of a hydroalcoholic mixture containing up to 60% v/v of alcohol, pre-heated at a temperature between 30 and 60 °C - forming a second solution, by adding said first solution and a second acid to said batch of water or hydroalcoholic mixture, operating in such a way that the levofolinate salt concentration in the second solution never exceeds 4% w/v, the pH value of the second solution always remains in the range between 3.0 and 4.5, and the temperature of the second solution always remains in the range between 30 and 60 °C - upon completion of the addition of said first solution to said batch of water or hydroalcoholic mixture, stirring the thus formed second solution while the precipitation of levofolinic acid occurs, maintaining the solution at a temperature between 30 and 60 °C, monitoring the pH and continuously adding said second acid to keep the pH stable, until it is observed that the pH value becomes stable with no need of further addition of said second acid, indicating the end of the crystallisation of levofolinic acid - recovering the crystalline levofolinic acid thus formed.
