Sample size calculation to detect mortality benefit of treatment

Hi folks,

I was wondering if you could help me with this. I thought I'd be able to work this out but my stats knowledge is very jaded now.

If the mortality rate from an illness is 2% and I want to conduct a trial comparing treatment X with placebo, with a view to finding out whether treatment X reduces mortality from that illness--- how do I determine what sample size I will need to be able to say whether or not treatment X has a mortality benefit.

Thanks in advance.