This paper describes a comprehensive experimental study of the extrudate swell behavior of high-density (HDPE) and linear low-density polyethylenes. The contributions from bulk melt elasticity, entry extensional flow, and exit boundary discontinuity are each explicitly investigated as a function of molecular weight and its distribution. The elusive apparent molecular weigh : dependence of the transient extrudate swell ratio (ESR) is elucidated to arise from the different molecular relaxation rates. A master curve is obtained at a fixed stress for the time-dependent ESR of different molecular weights when represented in terms of the rescaled time, t/tau, where t is the actual time allowed for growth of ESR and tau is related to the overall molecular relaxation time of each HDPE. The intricate influence of the exit boundary discontinuity on the extrudate swell is illustrated on the basis of the recently accumulated precise knowledge of wall slip behavior of linear polyethylenes (LPE). Wall slip induced by fluoropolymer coatings localized at the die exit wall is demonstrated to reduce the ESR of LPE under all stresses below an interfacial stick-slip transition. The entry effect is characterized in two ways : by comparing the ESR values below and above the stick-slip transition of the HDPE at the critical stress, and by comparing the ESR from a long die of aspect ratio L/D = 15 with that from an orifice die of LID = 1/4. The HDPE samples are found to exhibit an anomalously larger ESR above a critical stress at low temperatures (from 150 to 160 degrees C).