A plethora of work has been dedicated to the analysis of cell behavior on substrates with ordered topographical features. However, the natural cell microenvironment is characterized by biomechanical cues organized over multiple scales. Here, randomly rough, self-affinefractal surfaces are generated out of silicon,where roughness R_a and fractal dimension D_f are independently controlled. The proliferation rates, the formation of adhesion structures and the morphology of 3T3 murine fibroblasts are monitored over six different substrates. The proliferation rate is maximized on surfaces with moderate roughness (R_a ~ 40 nm) and large fractal dimension (D_f ~ 2.4); whereas adhesion structures are wider and more stable on substrates with higher roughness (R_a ~ 50 nm) and lower fractal dimension (D_f ~ 2.2). Higher proliferation occurson substrates exhibiting densely packed and sharp peaks, whereas more regular ridges favor adhesion. These results suggest that randomly roughtopographies can selectively modulate cell behavior.