We consider the sideways heat equation u xx (x, t) = u t (x, t), 0 x < 1, t 0. The solution u(x, t) on the boundary x = 1 is a known function g(t). This is an ill-posed problem, since the solution-if it exists-does not depend continuously on the boundary, i.e., small changes on the boundary may result in big changes in the solution. In this paper, we shall use the multi-resolution method based on the Shannon MRA to obtain a well-posed approximating problem and obtain an estimate for the difference between the exact solution and the solution of the approximating problem defined in V j .