Neuroblastomas are malignant, embryonic tumors, and most are probably congenital in origin. Neuroblastoma is a heterogeneous neoplastic disease with respect to response to treatment and prognosis. Some of its clinical behavior is ascribed to the ability of the tumor to regress spontaneously. Spontaneous regression of neuroblastoma is occasionally observed in young infants, but occurs extremely rarely in older children. This feature is in accordance with the concept that tumors develop by a series of changes, progressing from a dependent state to an autonomous state. Spontaneous regression was documented in less than 2% of Danish neuroblastoma patients. However, the "true" incidence of regression may be higher. Epidemiological data suggest that, in recent decades, borderline lesions may be included among truly malignant neuroblastomas. The recent birth cohorts compiled in Denmark show that 1 in 7,000 live births will develop neuroblastoma before 15 years of age. If 65% of all childhood neuroblastomas could be detected at or before the age of 6 months, then the expected prevalence by screening would be 1/11,000. One can speculate that 1/18,000 live births might possibly benefit from screening at age 6 months, since 59% of these children had tumors in stages III and IV diagnosed after the age of 6 months. However, the proof that screening can only detect lesions that would have progressed to become malignant tumors depends on observing an appropriate fall in the incidence of the malignant tumors in older children and a decline in mortality. Therefore, it is of some concern that the incidence of neuroblastomas appears to actually increase with screening while mortality has been little effected.(ABSTRACT TRUNCATED AT 250 WORDS)