We studied 153 patients with tumors digitally localized to 1 prostatic lobe with transrectal ultrasound and bilateral biopsy. Of these patients 65 (42%) had tumor in the clinically benign lobe as well as the suspicious lobe. These patients had higher serum prostate specific antigen (PSA) levels (27.7 +/- 28.1 versus 14.3 +/- 16.7 ng./ml., p = 0.0001) than those with negative contralateral biopsies. Radical prostatectomy was done in 57 patients; the 25 with positive bilateral biopsies had larger tumors (6.3 +/- 6.0 versus 2.5 +/- 2.4 cc, p = 0.0008) and a much higher likelihood of capsular penetration into the periprostatic fat (72% versus 28%, p = 0.0025) than the 32 with unilaterally positive biopsies. Patients with bilaterally positive biopsies also were more likely to have nodal disease (8% versus 0%), seminal vesicle invasion (20% versus 6%), positive margins (32% versus 19%) and biochemical (PSA) evidence of recurrence (20% versus 3%), although none of these differences was statistically significant. Of the 25 patients with bilaterally positive biopsies 12 (48%) had nonpalpable extension of tumor into the other lobe; adverse findings and outcomes were concentrated in these patients as opposed to the 13 patients in whom small incidental tumors were sampled by biopsy. Finally, 2 of the 25 patients (8%) with bilaterally positive biopsies had a positive surgical margin in the area of the contralateral neurovascular bundle; this was not observed in any of the 32 patients with unilaterally positive biopsies. These findings demonstrate that contralateral negative biopsies in patients with unilaterally palpable disease predict low volume, localized tumor and a negligible likelihood of surgical margin compromise when using a contralateral nerve-sparing approach. Bilaterally positive biopsies suggest larger tumor volume with a greater likelihood of adverse pathological findings and recurrence.