The Multidimensional hierarchical toolkit or Multi-Dimensional and Hierarchical (MDH) Database Toolkit is a Linux-based, open-sourced, toolkit of portable software that supports very fast, flexible, multi-dimensional and hierarchical storage, retrieval and manipulation of information in databases ranging in size up to 256 terabytes. The package is written in C and C++ and is available under the GNU GPL/LGPL/Free Documentation licenses in source code form. The distribution kit contains demonstration implementations of network-capable, interactive text and sequence retrieval tools that function with very large genomic data bases and illustrate the toolkit's capability to manipulate massive data sets of genomic information.