The first total synthesis of the novel hepatotoxin (+/-)-cylindrospermopson (1) has been accomplished in 20 steps from 4-methoxy-3-methylpyridine (12) in 3.5% overall yield. The substituted piperidine A ring 19 was generated stereospecifically by a four-step sequence using the addition of trimethylsilylethynylmagnesium bromide to 12 to give 16 and stereospecific addition of vinylcuprate to 16 to form 17. The reaction of diamine 26 with cyanogen bromide produced the cyclic guanidine C ring of 27. The key step in the synthesis was bromination of ketone 31, followed by hydrogenation to liberate the free guanidine, which underwent an intramolecular S(N)2 reaction to form the tetrahydropyrimidine ring B of 32. Further hydrogenation reduced the ketone to yield 42% of 32 containing the fully functionalized tricyclic system and protected hydroxymethyluracil side chain of cylindrospermopsin. Hydrolysis of the pyrimidine in concentrated hydrochloric acid and selective monosulfation completed the synthesis of cylindrospermopsin.