An experimental study was performed for the dispersion of highly viscous drops in turbulent flow in an agitated vessel equipped with a standard type of Rushton turbine impeller. Silicon oil-tetrabromoethane mixtures with different viscosities were dispersed in water. The viscosity and volume fraction of the dispersed phase ranged from 0.0107 Pas to 12.6 Pas and from 0.002 to 0.03, respectively. The transient size distributions of drops were measured at various impeller speeds after the initiation of agitation, and the time-variation of Sauter mean drop sizes were obtained. First, the relationship between the volume fraction and the final drop size was examined. The final drop size increased with increasing volume fraction due to the effect of drop coalescence. The variation of the final drop size with volume fraction was successfully expressed by a simple empirical equation. By the introduction of the empirical equation into the previous correlation for the final drop sizes, which was obtained in the absence of drop coalescence, all the experimental results could be expressed well. Subsequently, the relationships between the mean drop size and the agitation time were considered. Based on a simple analysis of the experimental data, a non-dimensional correlation was obtained for the time-variation of mean drop sizes. The validity of the correlation was examined by using both the present data for high-viscosity drops and the data obtained from a paper concerning low-viscosity drops. Consequently, the correlation was found to be applicable over a wide range of drop viscosities.