Transmethylation disorders are group of metabolic diseases related to the methionine cycle. It is a common name for four enzymes that cause inadequate methylation and, thus, primarily affect three organs: brain, liver and muscles. The third enzyme in metabolic reaction of methionine is S-adenosylhomocysteine hydrolase. Lack of activity of this enzyme is the disease which was first described by Croatian researchers in 2004. Up to now, only 10 cases of this disease have been reported. The illness is very difficult to be diagnosed by MDs, as many are unfamiliar with it. However, elevated levels of creatine kinase, accompanied by myopathy, are the clearest signs for suspecting this particular form of transmethylation disorder. Diagnosis is made by measuring S-adenosylhomocysteine and S-adenosylmethionine. Therapy is not etiological, but the disease can usually be controlled by an adequate diet. It is important to start a low-protein diet as early as possible in order to avoid the irreversible effects of AdoHcy's toxicity on the brain and locomotor system. The main goal is to prevent neuromotor retardation. Prognosis is better and complications are less severe if the disease is detected earlier. Complications are usually associated with thromboembolic incidents, hemorrhagic diathesis and liver pathology. Patients need a multidisciplinary approach and lifelong follow-up in order to control disease and prevent complications.