Iloprost is a chemically stable analog of prostaglandin I2 showing the same properties as the naturally occurring substance, but with advantages of ease of handling and administration to patients. A double blind within patient comparison of intravenous iloprost and placebo was undertaken in 13 patients with Raynaud's phenomenon severe enough to warrant short term hospitalization for intravenous dilator therapy; thermography was used as one form of assessment. Our results, while showing improvements in frequency of Raynaud's attacks after iloprost compared with placebo, show no significant effects on other variables.