Throughout the world, the number of patients suffering from diabetes mellitus (DM) is increasing on a daily basis, probably due to change in lifestyle. DM is mainly classified as either insulin-dependent Type 1 or non-insulin-dependent Type 2, according to the definition of WHO. To treat DM, which has many severe complications, several types of insulin preparations and synthetic drugs for Type 1 and Type 2 DM, respectively, have been developed and are in clinical use. However, there are several problems concerning the insulin preparations and synthetic drugs, such as physical and mental pain due to daily insulin injections and defects involving side effects, respectively. Consequently, a new class of therapeutic agents is anticipated. For this purpose, vanadium-containing complexes are expected to treat or improve both types of DM by using unique characteristics of the transition metal. In this article, the current state of research on insulin-mimetic vanadium complexes are reviewed, with special focus on the paramagnetic vanadyl (+4 oxidation state of vanadium) complexes with different coordination modes. To analyse the blood glucose-lowering effects of the vanadyl complexes, new results on the organ distribution and pharmacokinetic analysis of the vanadyl state in the blood of rats are also described.