The phenomenon of steam-water flow instability in geothermal well based on Newton's second law is considered. The factor causing instability is the reduction of gravitational force that counteracts with an increase in flow rate, resulting in a decrease of mixture density with increasing flow rate. External pressure can effectively influence the stability in case of sufficiently rapid response to changes of flow rate. The specific of the development of instability in the geothermal well does not assume a sufficiently rapid reaction of bottom-hole pressure. In other words, bottom-hole pressure reaction cannot ensure flow stability. Reaction of wellhead pressure can be a stabilizing factor. The example of a typical well of Pauzhetka geothermal field shows, that inversion of output curves is associated with the stabilizing effect of the additional hydraulic resistance at the outlet. It is noted, that wells have extended sections with internal instability, which may be the source of oscillation of flow parameters. (C) 2016 Elsevier Ltd. All rights reserved.