The incidence of stage A (incidental) adenocarcinoma of the prostate in transurethral resection (TUR) specimens is approximately 16%. This paper discusses the criteria for differentiating stage A1 versus stage A2 tumor, based on tumor volume and grade. Both the short-term (4 year) and long-term (8-10 year) natural history of untreated stage A1 prostate cancer are examined. Options to follow patients expectantly are presented. These include digital rectal examination and transrectal ultrasound. Specific problems relating to analyzing transrectal ultrasounds in patients who have had a prior TUR are addressed. Also, the unique aspects of transrectal ultrasound for stage A1 disease as it relates to the location of the lesion are expanded upon. The third option in the management of stage A1 disease is to monitor serum prostate specific antigen (PSA) levels. Areas covered include the sensitivity and specificity of PSA in general, and, in specific, serum PSA levels following TUR for stage A1 disease as a predictor of residual tumor. New data on a small group of patients who underwent delayed radical prostatectomy following diagnosis of stage A1 disease, where PSA data was available, are presented. The rationale for following patients with stage A1 disease by monitoring their serum PSA levels is supported by data from a group of men with normally sized prostates, benign prostatic hyperplasia, or cancer where longitudinal serum PSA levels were available. Finally, the option of radical prostatectomy for stage A1 disease is put forth. Data include a study of a large group of radical prostatectomy specimens performed for stage A1 disease.(ABSTRACT TRUNCATED AT 250 WORDS)