Spontaneous regression and maturation of neuroblastoma (NB) are well documented and occur frequently in infants, including those detected by mass screening. To seek histologic clues for regression/maturation in mass-screened NB, clinicopathologic features of 12 tumors that were resected after 2 to 18 months of untreated observation were reviewed. Unobserved screened and age-matched unscreened patients were also studied. To evaluate the possible important role of apoptosis, apoptotic cells were detected by in situ deoxyribonucleic acid (DNA) nick end labeling and immunohistochemical stain for activated caspase-3. Nests with a varying degree of reduced cellularity ("less cellular" and "hypocellular" nests) were common in patients younger than 18 months of age, and were rare in older patients. Two characteristic cells, which have not been focused previously, were frequent, especially in the hypocellular nests. One showed amorphic eosinophilic cytoplasm with pyknotic nuclei and the other contained plump cytoplasm with well-maintained nuclei. These cells were also observed in 89% of the unobserved screened NBs and 79% of the age-matched unscreened patients with good outcome, whereas they could not be confirmed in any of the age-matched unscreened NBs with poor outcome. The amorphic and plump cells were negative for activated caspase-3 and in situ DNA nick end labeling. From these results, the authors hypothesize that these cells most likely represent a degenerative process, in either a state before the activation of caspase-3 or a caspase-independent form of cell death. The presence of less cellular and hypocellular nests with amorphic/plump cells may serve as one of the important clues in predicting tumor prognosis.