The Effort Index (EI) of the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) was developed to identify inadequate effort. Although researchers have examined its validity, the reliability of the EI has not been evaluated. The current study examined the temporal stability of the EI across 1 year in two independent samples of older adults. One sample consisted of 445 cognitively intact older adults (mean age = 72.89; 59% having 12–15 years of education) and the second sample consisted of 51 individuals diagnosed with amnestic Mild Cognitive Impairment (mean age = 82.41; 41% having 12–15 years of education). For both samples, the EI was found to have low stability (Spearman's ρ = .32–.36). When participants were divided into those whose EI stayed stable or improved versus those whose EI worsened (i.e., declining effort) on retesting, it was observed that individuals with lower baseline RBANS Total scores tended to worsen on the EI across time. Overall, the findings suggest low temporal stability of the EI in two geriatric samples. In particular, individuals with poorer cognition at baseline could present with poorer effort across time. These findings also suggest the need to further examine the temporal stability of other effort measures.