The present invention relates to a hydrate of a neuraminic acid compound of formula (I) which exhibits excellent sialidase inhibition activity and a crystalline form thereof; to pharmaceutical compositions (particularly an anti-influenza agent) containing said hydrate or said crystalline form as an active ingredient; to the use of said hydrate or said crystalline form in the preparation of said pharmaceutical composition; or to a method for the prevention or treatment of diseases (particularly influenza), which comprises administering a pharmacologically effective amount of said hydrate or said crystalline form to a warm-blooded animal (particularly a human) in need of such prevention or treatment. 
The neuraminic acid compound of formula (I) (hereinafter referred to as Compound (I)) has been disclosed in the specification of Japanese patent application publication number Hei-10-330373 (see also U.S. Pat.  No.  6,340,702).  Compound (I) exhibits excellent sialidase inhibition activity and would be expected to become a useful agent for the treatment and prevention of influenza.  However, in the practical use of Compound (I) as a pharmaceutical agent storage stability and ease of handling of Compound (I) have been required.