Raised urinary albumin excretion (UAE) is associated with an increased risk of cardiovascular disease in non-insulin-dependent diabetes mellitus (NIDDM). We have examined the role of endothelial dysfunction as a possible explanation for this association in 94 NIDDM patients by investigating UAE, new cardiovascular events, and plasma concentration of von Willebrand factor (vWF), an indicator of endothelial dysfunction. At baseline, 66 patients had normal UAE (less than 15 micrograms/min), which remained normal in 33 (group 1) and increased in 33 (to median 31.5 micrograms/min, group 2). In 28 patients, baseline UAE was abnormal (67.1 micrograms/min, group 3). Follow-up ranged between 9 and 53 months. vWF did not change in group 1 (median 128% at baseline and 123% at follow-up), but increased in group 2 (from 116 to 219%, p less than 0.0001) and group 3 (from 157 to 207%, p = 0.0005). Baseline level of and change in vWF were strongly related to the development of microalbuminuria (R2 = 0.60, p less than 0.0001), but cardiovascular risk factors were not (R2 = 0.14). Raised baseline UAE was associated with an increased risk of new cardiovascular events only in patients with vWF concentrations above the median (relative risk 3.66, 95% CI 1.3-11.9) and not in patients with lower vWF (0.19, 0.01-1.33). In addition, the cardiovascular risk associated with increased UAE was modified by low compared with high concentrations of serum high density lipoprotein cholesterol (2.86 [1.03-8.48] vs 0.15 [0.01-1.43]). Dysfunction of vascular endothelium may be a link between albuminuria and atherosclerotic cardiovascular disease in NIDDM.