Renal insufficiency is an independent powerful predictor of morbidity and mortality in a number of cardiovascular disorders. A number of retrospective analyses of large heart failure trials have shown that even mild renal insufficiency has powerful negative predictive power in patients with both mild and severe heart failure. We discuss the available data on the prognostic value of mild renal insufficiency in heart failure, as well as possible mechanisms of this phenomenon. Future research should focus on its possible pathophysiology.