R help - Jan 2005 - Wilcoxon rank sum test

Hi, 

This might be a general question: I am comparing two
groups using Wilcoxon rank sum test. However, the size
of the two groups differ a lot, e.g., one group has 20
and the other has 10000. Is the test still appropriate
given this huge disparity in size? If yes, what is the
alternative?

Thanks!

Yong