In vitro determination of 3H-thymidine labeling index (LI%) was carried out in craniopharyngiomas from 16 patients submitted to repeated surgical procedures. This study showed a poor correlation between LI% values and prognosis, but allowed to detect a high LI% variation in the same tumor investigated in serial different times. The spatial distribution of S-phase proliferative cells in squamous epithelium of cystic components suggests a rationale for intra-cavitary treatment of cystic and mixed craniopharingioma and may help explain its success.