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.