Abstract: A two-stage process for freeze-drying an aqueous solution of a dosage amount of cyclophosphamide to yield a hydrate of cyclophosphamide (referred to as "CPA" for brevity), comprises, in a first stage, freeze-drying a solution of CPA in combination with an excipient until the moisture content of the freeze-dried material is less than 2% by wt, based on the amount of anhydrous CPA present; and, in a second stage, rehydrating the freeze-dried material until the moisture content of the product is in the critical range of from about 2% to 7% by wt, based on the net wt of CPA product, it being essential for stability of the product that at least one sugar excipient be present. The process requires that a major amount by weight of the excipient(s) or all of it, be mannitol, optionally with an additional sugar and/or a carboxylic acid, and/or a buffer salt; and, the mannitol is present in an amount at least one-half (0.5 times) as mush as the CPA (anhyd) present.