  We investigate the chaoticity parameter $\lambda$ in two-pion interferometry
in an expanding boson gas model. The degree of Bose-Einstein condensation of
identical pions, density distributions, and Hanbury-Brown-Twiss (HBT)
correlation functions are calculated for the expanding gas within the
mean-field description with a harmonic oscillator potential. The results
indicate that a sources with thousands of identical pions may exhibit a degree
of Bose-Einstein condensation at the temperatures during the hadronic phase in
relativistic heavy-ion collisions. This finite condensation may decrease the
chaoticity parameter $\lambda$ in the two-pion interferometry measurements at
low pion pair momenta, but influence only slightly the $\lambda$ value at high
pion pair momentum.
