OBJECTIVES: Standard nursing interventions, especially bed-baths, in ICUs can lead to complications or adverse events defined as a physiologic change that can be life-threatening or that prolongs hospitalization. However, the frequency and type of these adverse events are rarely reported in the literature. The primary objective of our study was to describe the proportion of patients experiencing at least one serious adverse event during bed-bath. The secondary objectives were to determine the incidence of each type of serious adverse event and identify risk factors for these serious adverse events. DESIGN: Prospective multicenter observational study. SETTING: Twenty-four ICUs in France, Belgium, and Luxembourg. PATIENTS: The patients included in this study had been admitted to an ICU for less than 72 hours and required at least one of the following treatments: invasive ventilation, vasopressors, noninvasive ventilation, high-flow oxygen therapy. Serious adverse events were defined as cardiac arrest, accidental extubation, desaturation and/or mucus plugging/inhalation, hypotension and/or arrhythmia and/or agitation requiring therapeutic intervention, acute pain, accidental disconnection or dysfunction of equipment, and patient fall requiring additional assistance. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: The study included 253 patients from May 1, 2018, to July 31, 2018 in 24 ICUs, representing 1,529 nursing procedures. The mean Simplified Acute Physiology Score II was 54 ± 19. Nursing care was administered by an average of 2 ± 1 caregivers and lasted between 11 and 20 minutes. Of the 253 patients included, 142 (56%) experienced at least one serious adverse event. Of the 1,529 nursing procedures, 295 (19%) were complicated by at least one serious adverse event. In multivariate analysis, the factors associated with serious adverse event were as follows: presence of a specific protocol (p = 0.011); tracheostomy (p = 0.032); administration of opioids (p = 0.007); presence of a physician (p = 0.0004); duration of nursing care between 6 and 10 minutes (p = 0.003), duration of nursing care between 11 and 20 minutes (p = 0.005), duration of nursing care greater than 40 minutes (p = 0.04) with a reference duration of nursing care between 20 and 40 minutes. CONCLUSIONS: Serious adverse events were observed in one-half of patients and concerned one-fifth of nurses, confirming the need for caution. Further studies are needed to test systematic serious adverse event prevention strategies.