We report an accurate determination of the hydrodynamic boundary condition of simple liquids flowing on smooth hydrophobic surfaces using a dynamic surface force apparatus equipped with two independent subnanometer resolution sensors. The boundary slip observed is well defined and does not depend on the scale of investigation from one to several hundreds of nanometers, nor on shear rate up to 5 x 10(3)s(-1). The slip length of 20 nm is in good agreement with theory and numerical simulations concerning smooth nonwetting surfaces. These results disagree with previous data in the literature reporting very high boundary slip on similar systems. We discuss possible origins of large slip length on smooth hydrophobic surfaces due to their contamination by hydrophobic particles.