In this paper we analyze the signal-to-noise ratio (SNR) for surface coil magnetic resonance imaging at 1.5 T. We have applied the treatment of Hoult and Lauterbur to determine the factors that most affect coil performance. We have imaged lossy phantoms with 8-, 10-, and 14-cm-diam circular surface coils and compared the results to body and head coil images. Surface coils can improve SNR by a factor of 4 or more for regions close to the surface. Surface coils are effective for regions up to 6 cm deep in the head and about 12 cm deep in the body. Nonuniformity of image intensity is a necessary requirement for improved SNR in surface coils. Coil losses make only a small contribution to image noise compared to tissue losses at 1.5 T. Surface coils need not be placed in close contact with the patient at 1.5 T.