Context The anti-inflammatory role of adiponectin has prompted interest in a potential role in acute inflammatory conditions associated with critical illness. It is unclear whether a random adiponectin measurement adequately reflects the 24-h profile in critically ill patients. Objective To assess the temporal profile of total and high molecular weight (HMW) adiponectin and interleukin-6 (IL-6) in 15 critically ill patients. Design A prospective, observational study. Setting Level II intensive care unit in a metropolitan hospital. Patients or Other Participants Fifteen critically ill patients expected to stay in the ICU for longer than 48 h were eligible for enrolment. Intervention(s) None. Main Outcome Measure(s) Serial, hourly measurements of total and HMW adiponectin and IL-6. Results Over a 24-h period, total and HMW adiponectin display considerable within-patient variability (coefficient of variation 34% and 87% respectively) and show no trend over time. Averaging 2 or 3 continuous measures reduced within-patient variability of both total and HMW adiponectin by up to 50% compared to one measure. There was a negative correlation between serum glucose and adiponectin (total P = 0·016, HMW P = 0·039). No relationship existed between adiponectin and IL-6 (total P = 0·62, HMW P = 0·35). Conclusions Marked within-patient, hourly variability in total and HMW adiponectin is evident in critically ill patients. A random measurement may not be reflective of the 24-h profile in these patients. A negative correlation exists between adiponectin and blood glucose levels and a positive correlation between adiponectin and oxygen saturation. No clear relationship exists between adiponectin and IL-6.