A method for determining viscosity, .eta., of a fluid downhole, calls for performing a nuclear magnetic resonance (NMR) survey of the fluid; determining a longitudinal relaxation time, T.sub.1, and an apparent transverse relaxation time, T.sub.2app, for the fluid; forming a ratio R of T.sub.1/T.sub.2app for the fluid; and determining the viscosity, .eta., according to the ratio, R. A computer program product for implementing the method is provided.