Abstract. The lattice cohomology of a plumbed 3-manifold M associated with a connected negative definite plumbing graph is an important tool in the study of topological properties of M and in the comparison of the topological properties with analytic ones, whenever M is realized as complex analytic singularity link. By definition, its computation is based on the (Riemann-Roch) weights of the lattice points of Z s , where s is the number of vertices of the plumbing graph. The present article reduces the rank of this lattice to the number of 'bad' vertices of the graph. (Usually the geometry/topology of M is codified exactly by these 'bad' vertices via surgery or other constructions. Their number measures how far is the plumbing graph from a rational one, or, how far is M from an L-space.)The effect of the reduction appears also at the level of certain multivariable (topological Poincaré) series as well. Since from these series one can also read the Seiberg-Witten invariants, the Reduction Theorem provides new formulae for these invariants too.The reduction also implies the vanishing H q = 0 of the lattice cohomology for q ≥ ν, where ν is the number of 'bad' vertices. (This bound is sharp.)