As part of a comprehensive study on factors affecting flow boiling in horizontal tubes, local boiling heat transfer coefficients were measured for R- 134a evaporating inside the same 12.00 mm bore, plain tube for horizontal flow, vertical upflow and vertical downflow over a wide range of mass velocities, vapor qualities and heat flux for a direct comparison of the effects on tube orientation on heat transfer. The experiments show a significant effect of flow direction on local heat transfer coefficients with upflow giving significantly lower values and downflow yielding even lower values. A flashing/subcooling effect on the heat transfer process caused by the negative/positive pressure gradient for upflow/downflow, respectively, is proposed as an important new factor affecting flow boiling.