The drop breakup in a heterogeneous turbulent pipe flow developing downstream of an orifice is reported. A two-phase flow is considered of n-heptane drops in a water-glycerin solution at three distinct volume concentrations: phi(d) = 0 (single-phase flow), 0.1, and 0.2. The maximum pressure drop across the orifice, Delta P-max, is found to be a linear function of the kinetic energy of the mixture that weakly depends on phi(d). The continuous-phase velocity field is measured by means of high-speed particle image velocimetry in an optically homogeneous medium. The turbulence energy level and its spatial distribution are strongly influenced by both the continuous-phase viscosity and the presence of the dispersed phase. Drop breakup is driven by an inertial mechanism and is observed where the turbulence is the most intense. The breakup probability, the mean number of fragments, and the daughter-drop tauter diameter- are plotted against the global Weber number based on AP,,,-,. For concentrations up to 0.1, breakup statistics are close to those obtained with an isolated drop in a single-phase flow of water. For phi(d) = 0.2, the increase of breakup probability with the Weber number- becomes weaker because of the volume reduction of the region of intense turbulence. The evolution of breakup probability with the distance from the orifice indeed shows strong differences between the test cases, arising from modifications in the local structure of the heterogeneous turbulence field. (c) 2006 American Institute of Chemical Engineers AlChE J, 53: 56-68, 2007.