SummaryBased on findings that showed increased P-selectin expression on platelets and on choroidal microvessels of patients with insulin dependent diabetes mellitus (IDDM), we hypothesized that also plasma concentrations of circulating (c)P-selectin would be increased in these patients.The aim of this study was to compare the plasma levels of cP-selec-tin between non-smoking patients with IDDM, treated with an intensified insulin therapy, and healthy controls. The study design was prospective, cross-sectional and analyst-blinded. Subjects were matched individually for sex, age and body mass index. Plasma levels of cP-selectin and of von Willebrand antigen (vWF-Ag) were determined by enzyme linked immunoassays.Forty-two pairs were available for intergroup comparison. Median plasma concentrations of cP-selectin in patients with IDDM (285 ng/ml; interquartile range: 233-372) were on average 21% higher than those of controls (236 ng/ml; interquartile range: 175-296; p = 0.004). Also, median plasma levels of vWF-Ag were 10% higher in patients (96 U/dl; interquartile range: 82-127) than controls (87 U/dl; interquartile range: 70-104; p = 0.025). There was no correlation between plasma concentrations of cP-selectin and vWF-Ag levels in either group (p ώ0.05).In conclusion, our results of increased cP-selectin levels are in line with increased P-selectin expression on platelets and on choroidal microvessels found in patients with IDDM. In view of the currently developed small molecule inhibitors of cell adhesion molecules, these independent observations together may provide a sound rationale to select P-selectin as a target for treating or preventing IDDM-associated micro- or macrovascular complications.