Conformation of polymer chains strongly confined in a narrow channel space was Studied over a broad range of polymer volume fractions phi using lattice Monte Carlo simulations. The longitudinal component of the chain dimension decreased in a power law of similar to phi(-1) as phi exceeded the overlap volume fraction. The conformation changed from the one extended along the channel to a random coil. The change occurred without much overlap between adjacent chains. As the conformational transition was completed, the chains started to penetrate each other. Contraction of the chains became more gradual, and eventually the longitudinal component of the chain dimension approached that of the unconfined chains with the overall chain dimension being smaller than that of the unconfined chains. Predictions of the scaling theory were thus confirmed with additional detailed information on the state of confined chains in each regime of characteristic phi dependence of chain dimensions. (C) 2004 Elsevier Ltd. All rights reserved.