In Study 1, we evaluated preference stability across 4 preference-assessment methods for 6 individuals, 5 of whom had autism spectrum disorder and 1 of whom had traumatic brain injury. We also measured participants' problem behavior as a corollary measure during all assessment methods. The highest mean correlation coefficients and Kendall rank coefficients of concordance across administrations were observed for the paired-stimulus and multiple-stimulus-without-replacement methods. Lower correspondence across administrations was observed for the free-operant and response-restriction methods. Although differentially higher levels of problem behavior did not occur with a single method, lower levels were consistently observed with the free-operant method. During Study 2, we evaluated the implications of lower coefficients on reinforcer efficacy by comparing an initially identified and an immediately identified high-preference stimulus in a reinforcer assessment. Initially identified and immediately identified high-preference stimuli were equally effective reinforcers, suggesting that fluctuations in preference do not necessarily affect reinforcer efficacy in practice.