Serum methylmalonic acid (S-MMA) as a sensitive indicator of cobalamin deficiency was introduced more than 10 years ago. The use of this method for identifying patients with cobalamin deficiency reflects much higher prevalence figures than was previously thought. In this review, all major studies on the subject are analysed. The least common denominator that could explain the probably overrated prevalence figures is the deteriorated renal function accepted for inclusion in virtually all the studies. A strong association between S-MMA and S-creatinine, even within the normal range for creatinine. has become increasingly apparent. At present, it is impossible to estimate how much of elevated S-MMA comes from impaired renal function and how much comes from impaired cobalamin metabolism. Thus, the use of S-MMA as the sole indicator of cobalamin deficiency cannot be recommended.