When a strong signal is presented monaurally, listeners can easily lateralize the sound. However, if noise is added to both ears, there may be uncertainty as to which ear received the signal. This uncertainty was measured over a range of signal energies with perfectly correlated noise (N0) and with uncorrelated noise (NU). In the main experiment, the monaural signal occurred on each trial, and this signal was presented to either the right or the left ear by random determination during the single observation interval. Listeners responded “right” or “left.” Measures of signal detection were also secured with the monaural signal under release from masking (N0) and without such release (NU). With uncorrelated noise (NU), the listener requires only slightly greater signal energy (1–2 dB) in order to lateralize as well as he can detect. With correlated noise (N0), the psychometric function for lateralization is not only displaced considerably toward higher signal energies, relative to those required for detection, but the slope of the function for lateralization is smaller than that for detection. When a monaural signal is easily detected in uncorrelated noise, it is also easily lateralized. However, when the signal is strong enough to be readily detected with correlated noise, it is still poorly lateralized.