Investigated is the transient growth of discrete bubbles in saturation nucleate boiling of PF-5060 liquid on rough Cu surfaces at an applied heat flux of similar to 0.5 W/cm(2). The uniformly heated 10 x 10 mm surfaces have an average roughness, Ra = 0.039 mu m (smooth Cu), and 0.21-1.79 mu m. The transient growth rate, departure diameter and detachment frequency of the discrete vapor bubbles are determined from images captured at 210 fps using high-speed video camera. On smooth Cu, the bubble departure diameter and detachment frequency are 655 +/- 40 mu m and 31 +/- 4 Hz. On the other Cu surfaces (Ra = 0.21-1.79 mu m), the measured values of 438 +/- 17 gm and 38 +/- 3 Hz, respectively, are independent of surface roughness. Results, in conjunction with the experimental nucleate boiling curves, estimate the surface average density of active nucleation sites as a function of wall superheat. For smooth Cu, the density of active sites ranges from 100 to 2000 cm(-2), compared to 650-10,000 cm(-2) for the rough Cu surfaces. For all surfaces, the active nucleation sites density increases with increasing the wall superheat and/or surface roughness. (C) 2015 Elsevier Ltd. All rights reserved.