GelBandFitter is a computer program that uses non-linear regression techniques to fit mathematical functions to densitometry profiles of protein gels. This allows for improved quantification of gels with partially overlapping and potentially asymmetric protein bands. The program can also be used to analyze immunoblots with closely spaced bands. GelBandFitter was developed in Matlab and the source code and/or a Windows executable file can be downloaded at no cost to academic users from http://www.gelbandfitter.org.