Large systematic errors exist both in the magnitude and trends of the Keck correlation,(1) when it is scaled up from laboratory results to field applications. Data also show an unexpected transient effect in most wells, whereby changes in surface pressure of 100's to 1000's of psi are found to change the bottomhole pressure by only 10's or 100's of psi, respectively. The authors speculate on possible causes for this phenomenon, but conclude that additional work is needed for accurate predictions of bottomhole pressures during fracturing treatments.