Laboratory test reference values are collected from a group of individuals presumed to be healthy to serve as controls in diagnostic comparisons. Pediatric test reference values sometimes vary from those of adults and/or among children of different ages based on their development and growth. Therefore, it is crucial to obtain age-matched laboratory test reference values. As the acquisition of samples from healthy children has become extremely difficult in recent years, clinical reference ranges calculated from routine laboratory data of patients using a statistical method have been used in some institutions, including our own. In addition, clinical judgment has become increasingly important for interpreting laboratory tests that are relatively less common and/or show marked variation. This article reviews pediatric laboratory test values of markers that are variable at different ages, such as alkaline phosphatase (ALP), alpha-fetoprotein (AFP), and immunoglobulins. In general, the serum level of ALP in a pediatric population is two to three times higher than that of an adult population. Circulating AFP levels are markedly higher in neonates just after birth and then decrease exponentially, reaching close to the average adult value by late infancy. Immunoglobulins, except for IgG, increase after birth in infancy. In contrast, IgG levels drop in early infancy due to the reduction of maternal IgG, and then rise as a result of increased IgG production. As these laboratory test values significantly depend on the age, they should be used with caution. Furthermore, it is deemed desirable to apply newly developed laboratory tests that have been shown to be beneficial in adults to a pediatric clinical setting.