Background. Altered balance is a common and debilitating symptom of fibromyalgia. Previous studies have investigated balance under single-task conditions that do not reflect real-life situations. The present study evaluated the impact of a simultaneous cell-phone conversation on balance performance in a case-control cohort from Spain. Methods. A cross-sectional case-control study was performed in 34 women recruited from local self-help organizations and university facilities (n = 18 fibromyalgia cases; n = 16 healthy, pain-free controls). Participants performed the 30 s postural stability test, as implemented in the Biodex Balance System, under two conditions: (a) as a single task and (b) as a dual task, i.e., while holding a simultaneous cell-phone conversation with a technician. Intergroup differences in dual task costs were calculated. Findings. Compared with controls, women with fibromyalgia showed higher dual-task costs in balance variables, such as overall sway (pp value = 0.039) and anterior/posterior sway (p value = 0.007). In the dual-task condition, overall (p value = 0.004) and anterior/posterior (p value = 0.012) sway indices significantly decreased in women with fibromyalgia but not in controls. Interpretation. In women with fibromyalgia, balance performance was adversely impacted by the holding of a simultaneous cell-phone conversation. The inability to conduct two tasks simultaneously may be related to reduced attention and may increase the risk of falling in this population.