Despite advances in treatment, long-term outcome of patients with diffuse large B cell lymphoma (DLBCL) is no better today than reported in 1975. A recent study applying DNA microarray technology revealed that patients whose cancer related to patterns of genes expressed in germinal center lymphocytes responded more favorably to chemotherapy than patients whose cancer related to patterns of genes expressed in activated lymphocytes. cDNA and oligonucleotide microarrays are described, and their applications in cancer research are reviewed. In addition to DLBCL, microarray technology has been used to study several types of cancer. The applications of microarray technology are numerous and include profiling gene expression patterns in order to facilitate diagnosis and predict response to therapy; correlating patterns of gene expression with prognosis; and identifying genes and gene products that are associated with tumorigenic phenotype or with drug resistance, among other applications. Microarraytechnology has also been used in cell lines to correlate gene expression and chemotherapy response. Furthermore, microarray technology may provide a useful tool to examine the development of drug resistance in cancer and has recently been used to study changes in gene expression caused by activated c-Myc in primary human fibroblasts. Tissue microarrays are described. In addition to the amplification of limited tissue re sources, tissue microarrays have the advantages of limiting the variability associated with tissue processing and limiting the necessary amount of reagent. Tissue microarrays have been used to determine the frequencies of amplication of 3 major breast cancer genes and identify overexpression of ERBB2 mRNA; assess and compare gene amplification in benign prostatic hyperplasia, primary prostate carcinoma, recurrent prostate tumors, and metastatic tumors; compare aggressiveness of prostate carcinoma in 2 patient populations; and study gene amplification across various tumor types. Furthermore, DNA microarray and tissue microarray techniques can be combined to provide convergent evidence of findings and to examine different aspects of gene expression. DNA array technology may also be used to identify critical molecular targets or to identify the critical rate-limiting step in a cascade of genes under the influence of a mutated gene. The historical progression of goals of the National Cancer Institute is reviewed, as well as the economic impact of reduction in cancer-associated mortality. Future efforts should continue the investment in basic research and more effectively integrate it with clinical trials and with approaches to prevention and treatment.