In this paper an ill-posed problem for the heat equation is investigated. Solutions u to the equation u, -u xx = 0, which are approximately known on the positive half-axis t = 0 and on some vertical lines x -xj,... ,x = x n , are considered and stability estimates of these solutions are presented. We assume an a priori bound, governing the heat flow across the boundary x = 0. 1980 Mathematics subject classification (Amer. Math. Soc): 35 K 05.