Caregivers' stress following a family member's stroke is likely accentuated by its associated uncertainty. The purpose of the current study was to examine the effect of uncertainty on caregivers' perceived and physiological stress (i.e., salivary cortisol). A prospective, longitudinal observational study was conducted with a convenience sample of 40 caregivers and stroke survivors recruited from acute care settings. Linear mixed models were used. Greater uncertainty was associated with higher perceived stress (p < 0.001), but not with physiological stress (p = 0.32 on waking, p = 0.06 evening), over the first 6 weeks post-stroke. A significant association between uncertainty and evening salivary cortisol level was found at 6 weeks post-stroke (p = 0.009). Recognition of uncertainty early in the caregiving period and targeted interventions may be useful in reducing perceived stress for this group. [Journal of Gerontological Nursing, 43(11), 30-40.].