BACKGROUND: The ratio between urinary albumin concentration (UAC) and urinary creatinine concentration (UCC) is widely used to estimate renal involvement. We examined how UAC and UCC associate with each other, with other risk factors, and with diabetic complications in a population-based sample of Type 2 diabetic patients. METHODS: A freshly voided morning urine specimen was provided by 1,284 consecutive, newly diagnosed diabetic patients aged 40 years or over in general practice. Albumin was measured by a polyethyleneglycol radioimmunoassay and creatinine by a modified Jaffe method. RESULTS: In a multivariate model including UAC, UCC, age, sex, HbA1c, and urinary glucose concentration, UAC increased with both age (P=.042) and HbA1c (P=.014), while UCC decreased (P<.001 and P<.001, respectively). In two regression models, the prevalence of diabetic retinopathy (P<.001) and relatively high resting heart rate (P<.001) increased with increasing UAC but decreased with increasing UCC (P=.002 and P=.005, respectively). CONCLUSION: The use of albumin/creatinine ratio (ACR) may introduce bias of unpredictable size and direction in comparisons of ACR with variables that are associated with UCC in their own right. In daily clinical practice, renal involvement in the individual patient can be estimated reliably with UAC or ACR measured in a freshly voided morning urine specimen, especially when considered together. However, the associations of the combined measure ACR should be interpreted with great caution in clinical and epidemiological research.