Wavy structure of downward annular gas-liquid flow with liquid entrainment in 15 mm pipe was studied using high-speed laser-induced fluorescence technique. Measurements were performed near the inlet, which was organized as a tangential slot. Spatiotemporal records of film thickness were obtained over the first 100 mm of the pipe length in order to investigate formation and initial stages of development of the disturbance waves. It was shown that for high enough gas and liquid flow rates disturbance waves appear and start to dominate in the wavy structure of liquid film within the area of interrogation. Disturbance waves were found to be formed due to coalescence of small high-frequency waves appearing at the inlet. Similar mechanisms of formation of large waves with fast ripples on them were observed far downstream for waves near transition to entrainment and for ephemeral waves in flow regimes with entrainment. Significant individual acceleration of disturbance waves at the initial stage of their development was observed. Spectral analysis has shown strong energy transfer from high to low frequencies, which is in agreement to the proposed mechanism of waves formation. (C) 2015 Elsevier Ltd. All rights reserved.