The creatine kinase (CK) is an enzyme catalizing the reversible reaction between creatine (Cr) and adenosine triphosphate (ATP) that results in the formation of phosphocreatine (PCr) and adenosine diphosphate. Reverse reaction, one in which creatine phosphate transfers a high-energy phosphate to ADP, allows a timely regeneration of ATP in tissues with high and fluctuating energy demands such as the skeletal and cardiac muscle, as well as the brain. Laboratory measurement of the creatine kinase activity is a simple, non-invasive and easily reproducible test. In most cases, increased activity of creatine kinase in children is accompanied with symptoms and signs indicating muscle damage or disease. A large number of various hereditary muscle diseases begin during childhood. Knowledge of the clinical features, patterns of inheritance and expected values of the laboratory parameters such as CK activity may greatly contribute to a timely and correct diagnosis of such diseases which is considered of the upmost importance for providing quality patient care. Clinicians should also take in account other, acquired causes of increased activity of creatine kinase. This thesis provides an overview of childhood autoimmune disease affecting the skeletal muscle, as well as an overview of scientific knowledge gained in recent studies concerning the complexity of possible causes of rhabdomyolysis in paediatric population. This pertains, but is not limited, to infective agents, electrolyte and endocrine disorders, adverse affects to medication, effects of toxins, trauma and more. Part of the thesis is dedicated to conditions which lead to falsely increased activity of creatine kinase, and there is also a mention of a condition widely known as idiopathic hyperCKemia, where the underlaying cause to elevated creatine kinase activity cannot be found despite extensive diagnostic evaluation.