The stability of cognitive performance in four schizophrenia patients was examined over a short time period. A cognitive battery highly sensitive to organismic state (the Repeatable Cognitive-Perceptual-Motor Battery, RCPM) was utilized. Single-case statistical methods tested for within-individual change across five assessments at two week intervals. All four subjects showed statistically significant post-baseline variation in a global measure of impairment (average impairment rating score) and in at least 50% of the individual tests having adequate test-retest reliability. Data are interpreted as providing suggestive evidence of transient fluctuations in performance within a stable window of impaired cognitive functioning. These fluctuations appear substantial enough to influence both research and clinical outcomes. The benefits of the single-case methodology employed are discussed. The data suggest that significant transient fluctuations in cognitive performance can be observed in schizophrenia in domains purported to represent vulnerability markers of the disorder. The importance of accounting for these transient fluctuations in the attempt to identify enduring, trait-like characteristics of cognitive functioning in schizophrenia is discussed.