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ASTRONOMY AND ASTROPHYSICS First evidence of the nite horizontal extent of the optical turbulence layers. Implications for the new adaptive optics techniques. Abstract. It was recently shown (Masciadri et al. 2000, Masciadri 2001a), using a numerical simulation (Meso-Nh model), that the hypothesis of considering horizontally uniform C 2 N can have severe consequences. For example, the integration of the C 2 N along lines of sight diierent from the zenith gives variations that can be larger than 0.50 00. In this paper we try to validate this result following two approaches. (a) numerical approach: we compare measured and simulated C 2 N prooles. The rst ones were obtained with a Generalized Scidar (GS) and the latter are simulated with the Meso-Nh model at the same az-imuth and elevation as those of the observations. Further measurements (DIMM, mast and balloons) are considered in order to support the comparison. (b) experimental approach: we compare C 2 N prooles measured (almost simultaneously) by a GS along diierent lines of sight. The results of this study show that (1) the horizontal size of the turbulent layers can be nite, (2) the simulations and the measurements are well correlated and (3) for the rst time, we show that the model can reproduce observed seeing values that vary as much as 0.50 00 during the same night. This deenitely shows that the numerical simulations are a useful tool in the context of the turbulence characterization for astronomical applications. Finally, we discuss the implications that a nite horizontal size of the turbulent layers could have on the new adaptive optics techniques, particularly in applications to the extremely large size telescopes.