Although there is renewed interest in prostate brachytherapy, little information is available on the effect of the procedure on prostate-specific antigen (PSA) changes over time. This study describes PSA kinetics after iodine-125 (I-125) transrectal ultrasound-guided transperineal implantation of the prostate. From February 1991-September 1997, 207 patients were treated with an I-125 prostate implant alone for T1-T2 prostate cancer. PSA values were obtained prior to treatment and at 1-73 months (median, 24 months). The change in PSA after implantation of the prostate was measured as a fraction of the pretreatment PSA (PSA at follow-up/pretreatment PSA). PSA failure was defined as two elevations in PSA or PSA > 1 ng/ml. One hundred fifty-five patients had PSA values recorded at the 1-month time period. A PSA value greater than the pretreatment PSA at 1 month was found in 27% (42/155). This had no significant effect on future PSA failure. The median percentage change in PSA after implantation for all patients were as follows: 1 month, 0.73; 3 months, 0.30; 6 months, 0.18; 12 months, 0.12; 18 months, 0.12; 24 months, 0.08; 30 months, 0.07; 36 months, 0.08; 42 months, 0.08; and 48 months, 0.05. The most significant decline occurred in the first 12 months. This was followed by a more gradual decline between 12-24 months. There was little change in PSA values after 24 months. The 1-year PSA value had a significant effect on PSA failure. Patients with a 1-year PSA <1 ng/ml (66) had an actuarial 4-year freedom-from-failure rate of 90%, compared to a rate of 62% for those with values >1 ng/ml (69) (P = 0.002). Twenty-seven patients developed PSA failure. The time to PSA failure ranged from 12-48 months (median, 24 months), but most (20/27) failures occurred after 18 months. We conclude that the greatest decline in PSA after I-125 implantation of the prostate occurs during the first year, and little change occurs after 2 years. A 1-year PSA value > 1 ng/ml is highly predictive of eventual PSA failure, which occurs in most patients after 18 months posttreatment.