The prognostic value of modal deoxyribonucleic acid in low grade, low stage untreated prostate cancer. We selected for a prospective surveillance study 167 patients with untreated grades 1 and 2 prostate cancer. The tumors were classified regarding modal deoxyribonucleic acid value (ploidy) by flow cytometry, cytological grade by transrectal fine needle aspiration biopsy and local tumor stage. Of the patients 146 could be evaluated. Mean followup was 50 months. The initial ploidy was statistically correlated to cytological grade but not to initial local tumor stage, prostatic acid phosphatase activity or patient age. Initial ploidy and cytological grade had a prognostic value regarding local tumor progression when considered as single predictors and in combination. Two patients with diploid and 8 with nondiploid tumors initially had metastases during the surveillance. Five patients (1 with diploid and 4 with nondiploid disease) died of prostatic cancer. Modal deoxyribonucleic acid value and cytological grade were of prognostic value in untreated prostate cancer. 