This study compares the frequency of prominent nucleoli in low-grade adenocarcinoma with that of its frequent mimicker, adenosis. One hundred thirteen transurethral resection specimens of stage A purely low-grade adenocarcinoma (only Gleason score 1 or 2) were evaluated. Eighteen cases of adenosis were evaluated for comparison. Prominent nucleoli were defined as those with a greatest dimension more than 1.6 microns as measured with an ocular micrometer. The frequency of prominent nucleoli in each focus was estimated as (1) none, (2) rare (< 5% of epithelial cells), (3) occasional (5% to 50% of epithelial cells), and (4) frequent (> 50% of epithelial cells). Twenty percent of cases of adenocarcinoma had, at most, rare prominent nucleoli. Eight percent of adenocarcinoma cases had no prominent nucleoli. Twenty-eight percent of cases of adenosis had at least one focus of occasional or frequent prominent nucleoli. We conclude that a small but significant subset of low-grade prostatic adenocarcinomas lack prominent nucleoli and, likewise, a significant proportion of cases of adenosis have prominent nucleoli. Like many other histologic features of these lesions, we feel there is a spectrum of frequency of prominent nucleoli, with overlap between the two. The significance of nucleoli should be taken in context with other cytologic and architectural features characteristic of prostatic adenocarcinoma and adenosis. In difficult cases basal cell-specific immunohistochemical stains may be helpful.