When two or more rf coils are used to receive NMR signals from the same part of the patient, the overall sensitivity of an image formed from some combination of these signals may be degraded if noise voltages in the coils are correlated. Whereas it is straightforward to calculate correlation caused by noise sources in the coils themselves, there is disagreement about noise correlation when that noise arises from thermally driven currents within the body itself. A general analysis based on the principles of reciprocity, superposition, and energy conservation yields an equation for sample-induced noise correlation in terms of circuit impedances. Furthermore, the analysis yields a very simple expression for the total noise correlation (i.e., noise from patient and coil elements) between the coils. This latter expression is applicable to any two-port receiver coil assembly, including single coils with two modes, such as a birdcage coil, and in general, to any two ports of an n-port receiver system. © 1992 Academic Press, Inc.