Recently, the incidence of perinatally detected asymptomatic adrenal gland masses has increased because of widespread use of radiological diagnostic tools. However, optimal treatment of these masses has not been determined. The aim of this study is to elucidate the treatment guideline of perinatally diagnosed adrenal gland masses. The authors retrospectively reviewed the medical records of the 11 patients with asymptomatic adrenal gland mass, detected perinatally, between 1999 and 2004. Six cases were detected by prenatal ultrasound and 5 cases were incidentally detected by postnatal ultrasound. Six patients (surgery group) underwent mass excision. The pathologic diagnoses were neuroblastoma (n=4), adrenocortical adenoma (n=1) and adrenal pseudocyst (n=1). The indications for operation were suspicion of neuroblastoma (n=5) or absence of size decrease during observation (n=1). Three of the 5 suspicious cases of neuroblastoma and one case under observation were proven to be neuroblastoma. There was no surgical complication in the urgery group. All neuroblastoma patients have been well during the follow up period ($24.4{pm}14.4$ month) without evidence of recurrence. Five cases (observation group) were closely observed because of the benign possibility or size decrease in follow up ultrasound. During the observation period ($39{pm}21$ week), 4 cases showed complete spontaneous resolution and 1 case showed markedly decreased size of the mass but could not be followed up completely. Surgical resection of the perinatally diagnosed asymptomatic adrenal gland mass is a safe treatment method especially in case of suspicion of neuroblastoma, but closed observation can be applied.