ABSTRACT: Cognitive dysfunction is a common fibromyalgia (FM) symptom and can impact on the daily lives of those affected. We investigated whether within-day pain intensity ratings were associated with contemporaneous objective and subjective measures of cognitive function and whether within-day increases in pain intensity preceded increases in cognitive dysfunction or vice versa. Inclusion of a non-FM group allowed us to examine whether effects were specific to FM. Fifty people with FM and 50 non-FM controls provided 7 days of data. Cognitive tests (processing speed and working memory) and ecological momentary assessments (pain intensity and self-reported cognitive functioning) were conducted ×5/day. Three-level multilevel models examined contemporaneous and within-day 1-lag pain intensity-cognitive functioning associations. Interaction terms assessed possible moderating effects of FM status. Momentary increase in pain was associated with increased self-reported cognitive dysfunction, more strongly so for those with FM (B = 0.27, 95% confidence interval 0.22-0.32; non-FM B = 0.17, 95% confidence interval 0.10-0.23). For the FM group, higher pain was associated with longer processing speed; for the non-FM group, higher pain was associated with shorter processing speed. Pain increase did not precede change in subjective or objective cognitive function in the FM group, but reduction in working memory preceded increase in pain intensity. This finding warrants further research attention and, if replicated, could hold prognostic and/or therapeutic potential.