Mortality in diabetes mellitus is related mainly to its renal and cardiovascular sequelae. Microalbuminuria identifies those at increased risk of both complications, possibly because microalbuminuria is a marker of generalised endothelial dysfunction. Screening for microalbuminuria is of proven benefit in insulin-dependent diabetes, as therapeutic strategies are available that are at least partially effective. Such benefits have not yet been demonstrated in non-insulin-dependent diabetes. Arguments are presented to suggest that microalbuminuria should be defined on the basis of three or more daytime or 24-h urine collections. This increases sensitivity compared to the use of overnight urine collections while specificity is maintained.