The expression levels of thousands of genes in different tissues or cells in different conditions can be detected all at one time by DNA microarray technology. A new, grid-layout method for the visualization results of hierarchical cluster analysis of DNA microarray data is proposed and incorporated in the Microarray Interactive Graphics Toolkit (MIGT). The grid-layout consists of a set of regular, two-dimensional grid units. Each unit represents a cluster or a group of gene clusters. The units are connected to adjacent ones by the neighborhood relation of the clusters in a hierarchical tree. Nodes lying near each other in the hierarchical tree are mapped onto nearby grid-layout units. The number of units may vary on a scale from a few dozen up to several thousands, depending on the number of the nodes in a hierarchical tree. Different colors are assigned to the units with RGB value according to the coordinates of the units, and the inter-distances, which are the distances between clusters in a hierarchical tree, and the intra-distances, which are the distances between genes within one cluster. The closer the inter-distances, the more similar the color of the units are, the smaller the intra-distances, the warmer the color of the unit is.