Objective To study the short-term effects on sublingual microcirculatory flow of high dose dexamethasone in critically ill patients. Setting/Patients In an observational prospective cohort pilot study patients were treated with dexamethasone (1 mg/kg). Patients treated with any steroid within seven days before inclusion were excluded. A sublingual sidestream dark field (SDF) measurement was taken before dexamethasone treatment and after 15, 30 and 60 minutes. The microvascular flow index (MFI) was determined (0 = no flow, 1 = intermittent flow, 2 = sluggish flow and 3 = normal flow). Measurements/Results The patients (n=21) had a median APACHE II score of 23. The median MFI improved from 2.58 (Inter Quartile Range, IQR, 0.65) to 2.92 (IQR 0.33) in the small vessels (p=0.004) and MFI also increased from 2.58 to 2.92 (p=0.06) in the medium sized vessels. The large vessels showed optimal flow anytime. The MFI improvement was, in a multivariate linear regression analysis unrelated to the use of vasoactive drugs or fluid balance. Conclusions The microcirculatory flow measured by sublingual SDF improves in the small vessels within the first hour of treatment in the ICU. The use of vaso-active medications and fluids was unrelated to this improvement. The administration of high dose dexamethasone was not associated with vasoconstrictive effects. © 2010, Nederlandse Vereniging voor Intensive Care. All Rights Reserved.