An experimental setup was designed to perform nucleate boiling experiments in upward saturated flow conditions, in order to investigate the influence of vertically aligned vapor bubble nucleation sites on one another. Experiments were performed by activating two bubble generators, of which the inter-site distance can be varied by steps of 2 mm. Depending on mass flow rate, flow direction and heat fluxes to both bubble generators, nucleation sites have been shown to interact at any nucleation site distance. The results have shown two major trends. The first trend is caused by additional convection (not by bubbles) from the upstream bubble generator, BG2, to the downstream bubble generator, BG1, increasing its nucleation frequency and bubble detachment diameter. The influence of additional convection on bubble frequency and diameter diminishes with increasing inter-site distance, S, and initial bubble nucleation frequency at BG1. The influence of added convective heat is enhanced by increasing the liquid bulk flow rate. The second trend is seen when BG2 initiates its own bubble nucleation. Vapor bubbles that nucleate at BG2 and pass by BG1 in close proximity have an inhibitive effect on bubble nucleation at BG1. The effect of this inhibitive trend diminishes with increasing nucleation frequency at BG1. Because of the significance of the effect of hydrodynamic interaction on the number of active nucleation sites and bubble size at detachment, mechanistic modeling of nucleate flow boiling is expected to benefit from the quantifications presented in this study. (C) 2014 Elsevier Ltd. All rights reserved.