Robert J. Amato and Joan Hernandez-McClain Genitourinary Oncology Program, The Methodist Hospital Research Institute, Houston, TX. Abstract The objective of this Phase II clinical trial was to determine the effects of chronic GM-CSF dosing on PSA levels in men with hormone-refractory or hormone-naïve prostate cancer. Six hormone-refractory and 10 hormone-naïve patients were recruited from an institutional practice and were treated with 250 and 125 μg/m2 of GM-CSF, respectively, 3 times per week for continuous 12-week treatment cycles until evidence of disease progression, as indicated by 2 consecutive rising PSA levels. PSA levels were measured every 6 weeks. Of the 6 hormone-refractory patients, 2 were classified with progressive disease after 4 months and 1 after 1.75 months. The best PSA responses for the remaining 3 patients were 3%, 12%, and 32% declines which lasted from 1.75 to 8.5 months. Of the 10 hormone-naïve patients, 2 were classified with progressive disease after 3 and 12 months, and 1 patient met the criteria for stable disease after 7.75 months. The best PSA response for the remaining 7 patients ranged from 7% to 42% declines which lasted from 0.5 to 10 months. These results indicate that further study of GM-CSF administration is not warranted for hormone-refractory patients but is recommended for hormone-naïve patients using a chronic dosing regimen.