A fast-neutron-emitting radioisotope, 252Cf, is being tested in clinical trials of neutron brachytherapy for cervical cancer. The efficacy for histological eradication of bulky stage IB cervical tumors (mean diameter, ∼6 cm) using combined radiation and surgery was studied in 65 patients treated with 137Cs or 252Cf before surgery during 1983-1986. Forty-four patients were treated with 137Cs and 21 were treated with 252Cf at equivalent doses of radiation. Fifteen of the 44 specimens (34%) were positive after 137Cs therapy. Only one of the 21 specimens was positive after 252Cf therapy (P = .025), and that patient was treated in a delayed schedule 21 days after the start of external-beam irradiation rather than early in the course. 252Cf therapy required a much lower radiation dose and shorter treatment time. The study compared tumor destruction of an identically staged human cervical tumor in situ by direct histological means, using 252Cf neutron therapy or conventional photon therapy at an identical and equivalent dose adjusted by a relative biological effectiveness of 6.0 for 252Cf.