This paper documents some of the effects of free stream turbulence on the mean flow properties of turbulent boundary layers in zero pressure gradients. Attention is concentrated on flows for which the momentum thickness Reynolds number is less than about 2000. Direct Reynolds number effects are therefore significant and it is shown that such effects reduce as the level of free stream turbulence rises. A modification to Hancock’s [1] empirical correlation relating the fractional increase in skin friction at constant Reynolds number to a free stream turbulence parameter containing a dependence on both intensity and length scale is proposed. While this modification has the necessary characteristic of being a function of the free stream turbulence parameters as well as the Reynolds number, it is argued that the relative importance of intensity and length scale changes at low Reynolds numbers; the data are not inconsistent with this idea. The experiments cover the range 500 ⪝ Reθ ⪝ 2500, u′/Ue ⪝ 0.07, 0.8 ⪝ Le/δ ⪝ 2.9, where u′/Ue is the free stream turbulence intensity and Le/δ is the ratio of the dissipation length scale of the free stream turbulence to the 99 percent thickness of the boundary layer.