Experimental test results for flow boiling of pure ammonia inside horizontal tubes were obtained for a plain stainless steel tube. Tests were run at a nominal saturation temperature of 4°C, nine mass velocities from 20- 140 kg/m2s, vapor qualities from 1-99% and heat fluxes from 5-58 kW/m2. Two-phase flow observations showed that the current test data covered the following regimes: fully stratified, stratified-wavy, intermittent, annular and annular with partial dryout. The Kattan-Thome-Favrat flow boiling model accurately perdicted the local heat transfer coefficients measured in all these flow regimes with only two small modifications to their flow map (to extend its application to G < 100 kg/m2s). Their flow boiling model was also successfully compared to the earlier ammonia flow boiling data of Chaddock and Buzzard (1986). The Gungor-Winterton (1987) correlation instead gave very poor accuracy for ammonia.