Researchers have documented the consequences of relationship instability for parenting stress but have given little attention to within-partner relationship instability. In this study, the authors used data from the Fragile Families and Child Wellbeing Study (N = 3,544) to estimate the association between within-partner relationship instability (known as churning or on-again/off-again relationships) and parenting stress. First, they found that by the focal child's 5th birthday about 16% of biological parents experience churning. Second, compared to being stably together with or stably separated from the child's other parent, churning is associated with greater parenting stress for both mothers and fathers. Because parenting stress is the same or higher among churners compared to their counterparts who stably separate, this suggests that, more than a change in partner, relationship instability—whether within or across relationships—is tied to parenting stress.