We have experimentally observed that the optical absorption spectra of single-walled carbon nanotube ~SWCNT! films change drastically and reversibly by pressure up to 4.1 GPa. The main absorption feature at 0.68 eV attributed to the interband optical transition in semiconducting nanotubes @chiral (n,m) and zigzag (n,0), with n2mÞ3i # shifts to lower energy and eventually vanishes with increasing pressure. Accordingly, we infer that the electronic structure and the electronic properties of SWCNTs in the solid state are highly sensitive to intertube interactions and/or to symmetry breaking.