The purpose of this study was to investigate performance on the Random-Dot E (RDE) stereotest under binocular and monocular non-stereoscopic viewing conditions. Sixteen adult observers with normal vision were tested with the RDE stereotest. Four new RDE tests were shown to each observer in varying combinations of monocular viewing, binocular but non-stereoscopic viewing, and normal binocular viewing conditions. The test conditions were masked (where possible) and were presented in pseudo-random order. Control experiments were also conducted using the Frisby stereotest. Fifteen of the sixteen observers could discern some differences between the plates monocularly and therefore satisfied the passing criterion of the RDE test. Under these conditions, no observers could discern the stereoscopic E figure nor did any observer report a sensation of depth. No observer satisfied the passing criterion of the Frisby test monocularly. We conclude that caution should be used when interpreting results from the RDE stereotest, since adult observers could discern some differences between the test plates monocularly.