ImmuCo is a database of gene Co-expression and Correlation in Immune cells. The current version includes expression data for a total of 20,283 human and 20,963 mouse genes from the Affymetrix Human Genome U133 Plus 2.0 and Mouse Genome 430 2.0 microarrays, respectively, enabling co-expression and correlation analysis between any two genes. These arrays are from 11 human and 7 mouse cell types, including 18 human and 10 mouse cell groups. The signal values used for analysis were derived using the MAS 5.0 algorithm. Co-expression is reflected by the signal values and detection calls, whereas expression correlation and strength are reflected by a Pearson correlation coefficient. A scatter plot of the signal values is also provided to graphically illustrate the extent of correlation. ImmuCo supports requests using gene symbol (or alias), Entrez Gene ID, and probe set ID. Currently, 20,283 human and 20,963 mouse genes can be queried.