A number of field and laboratory studies have been carried out to accurately predict the effect of drillstring rotation on downhole pressure and equivalent circulating density (ECD). Field studies indicated that drillstring rotation often results in an increased ECD. This is in contradiction with the results obtained from number of laboratory studies and other field studies. Consequently, there is no comprehensive model that accounts for the effect drillstring rotation on wellbore hydraulics. Recently, simple empirical models have been developed based on field measurements alone. Although these models can be very useful as they are based on field measurements, they have no physical basis and are limited to specific ranges of field parameters.This article presents results of field studies and theoretical analysis conducted on the effect of drillstring rotation on wellbore hydraulics. Field measurements during actual drilling operation were obtained from four different wells. Key drilling parameters such as flow rate, drillstring rotation speed, rate of penetration, ECD and return density, were recorded as a function of measured depth.Selected published field measurements were analyzed systematically using dimensional analysis techniques. After correlating different dimensionless groups, a new semi-empirical model was developed. The model was rigorously tested for its accuracy. Model predictions were compared with new field measurements and predictions of an existing model. Model predictions show good agreement with field measurements. The new model exhibits appreciably better accuracy than the existing one.The model developed in this investigation is relevant to manage ECD in slim holes, deepwater wells, and extended-reach wells where the increased wellbore length results in excessive pressure loss and limits the operating window for bottom hole pressure. In deepwater applications, ECD management becomes critical due to the narrow operating window between the pore and fracture pressure gradients.