To define the spectrum of pilomyxoid morphology and to characterize the association between pilomyxoid astrocytoma (PMA) and pilocytic astrocytoma (PA), 84 cases of pediatric astrocytomas with pilomyxoid features were reviewed. Forty-two of these tumors had coexistent features of PMA and PA ("intermediate"). With the accumulation of more cytoplasm, fibrillar background, microcysts, and thickened blood vessels, these intermediate tumors were more PA-like and less like classic PMA. In the recurrent specimens, PMAs and intermediate tumors sometimes showed more prominent PA features. Both PMA and intermediate tumors involved sites outside the hypothalamus, optic chiasm, and the third ventricle. Both the existence of intermediate tumors and the finding that some PMAs and intermediate tumors mature into PA-like neoplasms over time, provided strong support for a biological relationship between PMA and PA. Additional evidence for a "maturational effect" was the finding that intermediate tumors occurred in patients with a median age of 36 months compared with the median age of 21 months for those with PMAs (P=0.017). Features that were often assumed to be poor prognostic indicators in gliomas, that is, necrosis, mitotic figures, and vascular proliferation, were not uncommon in typical PMAs and intermediate lesions. Further follow-up is needed to more accurately determine the prognosis of intermediate tumors.