OBJECTIVE Microalbuminuria is associated with cardiovascular mortality in subjects with non-insulin-dependent diabetes mellitus (NIDDM). However, little is known about this association in nondiabetic subjects. Specifically, it is not known whether microalbuminuria is related to an early stage of atherosclerosis manifested as increased intima-media thickness (IMT) of carotid arteries. We investigated the relationship between microalbuminuria and carotid artery IMT in 991 nondiabetic and 450 NIDDM subjects aged 40 to 69 years. METHODS Microalbuminuria was defined as albumin-to-creatinine ratio > or = 2 mg/mmol in a morning spot urine sample. B-mode ultrasound was used to assess the IMT of the common and internal carotid arteries. RESULTS Altogether 13.9% of nondiabetic and 27.6% of NIDDM subjects had microalbuminuria, and 31.1% of nondiabetic and 50.8% of NIDDM subjects had hypertension. Subjects with microalbuminuria had greater common carotid artery (CCA) IMT than those without microalbuminuria (nondiabetic: 0.84 +/- 0.02 versus 0.80 +/- 0.01 mm, P = .010; NIDDM: 0.89 +/- 0.02 versus 0.86 +/- 0.01 mm, P = .152; combined: 0.86 +/- 0.01 versus 0.82 +/- 0.01, P = .005). The association of microalbuminuria and CCA IMT was independent of age, sex, ethnicity, smoking, and lipoprotein levels. Although further adjustment for hypertension in the multivariate linear regression analysis attenuated the difference in CCA IMT between subjects with and without microalbuminuria, this difference continued to be significant (combined: 0.86 +/- 0.01 versus 0.83 +/- 0.01, P = .015). In contrast to CCA IMT, microalbuminuria was not related to ICA IMT. CONCLUSIONS Microalbuminuria was associated with increased CCA IMT. This relationship was only partly mediated by hypertension. Thus, microalbuminuria is related to atherosclerosis at an early stage of the disease process.