A new modified form of the equation of N. Phan-Thien and D. C. Pham (J. Non-Newtonian Fluid Mech. 72, 305 (1997)) is proposed to describe the viscosity-concentration behavior of emulsions of nearly spherical droplets. The proposed equation, as well as other existing theoretical equations, is evaluated in light of a large body of experimental data on concentrated emulsions, covering a broad range of dispersed-phase to continuous-phase viscosity ratios (4.15 x 10(-3) to 1.17 x 10(3)). In general, the experimental data exhibit large deviations from the existing theoretical equations; for example, the theoretical equation of Phan-Thien and Pham underpredicts the relative viscosity of concentrated emulsions by a large amount. The equation proposed in this work describes the experimental viscosity data of different emulsion systems remarkably well.