  We provide a set of analytic fits to the radii of pre-mainsequence stars in
the mass range 0.1 < M/Msun < 8.0. We incorporate the formulae in N-body
cluster models for evolution from the beginning of pre-main sequence. In models
with 1,000 stars and high initial cluster densities, pre-mainsequence evolution
causes roughly twice the number of collisions between stars than in similar
models with evolution begun only from the zero-age main sequence. The
collisions are often all part of a runaway sequence that creates one relatively
massive star.
