We show that Bowen's equation, which characterises the Hausdorff dimension of
certain sets in terms of the topological pressure of an expanding conformal
map, applies in greater generality than has been heretofore established. In
particular, the property of uniform expansion may be significantly weakened to
positivity of the Lyapunov exponent. Among other things, this allows us to
compute the dimension spectrum for Lyapunov exponents for maps with parabolic
periodic points.