The linear and lowest order nonlinear response of a quantum well resonant tunneling diode is evaluated using quantum transport theory. The calculations show that the negative conductance persists up to about 5 THz, although parasitic circuit elements will limit the maximum oscillation frequency to a much lower value. The nonlinear response (rectification) remains significant to frequencies near 10 THz and shows a resonant peak near 4 THz. These calculations support the interpretation of the experimental data of T. C. L. G. Sollner, W. D. Goodhue, P. E. Tannenwald, C. D. Parker, and D. D. Peck [Appl. Phys. Lett.43, 588 (1983)] that rectification at 2.5 THz was observed in their devices.