RISCALW is a novel convenient Windows program for risk calculation in families with Duchenne muscular dystrophy. It is based on an extended genetic model which includes germline mosaicism and different new mutation rates depending on sex and mutation type. Arbitrary family structures and additional diagnostic information such as genotypes from intragenic and flanking genetic markers of the dystrophin gene, creatin kinase values, and female deletion test results can be taken into account. Comprehensive online help is available. RISCALW has been extensively tested in more than 100 families. The user is able to perform risk calculations in large families in an easy way, and thus explore the dependence of the results on different model assumptions, for example different mutation rates in males and females. A detailed handbook and example files are distributed with the program. RISCALW is freely available for scientific use at http://www.riscalw.uni-hd.de.