An experimental investigation has been made of the influence of the (initial) bubble size and bubble concentration distribution on the gaslift efficiency for a vertical air - water flow through a pipe. of 72-mm dia. and 18-m height Three significantly different bubble injectors were used. It was found that with decreasing bubble size, the liquid production increases at constant gas flow rate and, hence, the gaslift efficiency increases. Also, the initial concentration distribution can be significant, as this distribution can influence the coalescence of bubbles close to the injector. With decreasing bubble size, the transition from bubbly flow to slug flow can be postponed. This is one of the main reasons for the beneficial effect of the bubble size on gaslift efficiency. A theoretical model has been developed to predict the influence of the bubble size on the gaslift efficiency. The agreement is reasonable.