We examine variability observed in heat flow determinations collected on hot spot swells. We find substantial scatter in heat flow at wavelengths of a few hundred kilometers and less at both Hawaii and Reunion, where closely spaced data exist, and large variability in the regional heat flow surveys at Cape Verde, Bermuda, and Crozet. Our preferred interpretation is that the observed variability is due to fluid flow. The presence of fluid flow admits the possibility that some heat is lost through advection such that the mean observed heat flow is less than the actual mantle flux. If so, the full magnitude of sublithospheric thermal variations may not be observed. This interpretation has important implications for understanding heat flow determinations made on hot spot swells and resulting geodynamic inferences. We suggest fluid flow may be masking variations in sublithospheric heat flux making available surface heat flow values a poor discriminator between geodynamic models for hot spot swells. Future field programs, methods of data analysis, and models should be designed to help extract a low-frequency mantle flux disguised by a high-noise surface filter.