The wide-ranging biologic malignancy of prostate cancer is strongly correlated with its extensive and diverse morphologic appearances. Histologic grading is a valuable research tool that could and should be used more extensively and systematically in patient care. It can improve clinical staging, as outlined by Oesterling et al (J Urol 138: 92-98, 1987), during selection of patients for possible prostatectomy by helping to identify the optimal treatment. Some of the recurrent practical problems with grading (reproducibility, "undergrading" of biopsies, and "lumping" of grades) are discussed and recommendations are made. The newer technologically sophisticated but single-parameter tumor measurements are compared with one important advantage of histologic grading: the ability to encompass the entire low to high range of malignancy. The predictive success of grading suggests that prostate cancers have more or less fixed degrees of malignancy and growth rates (a hypothesis of "biologic determinism") rather than a steady increase in malignancy with time. Most of the observed facts can be interpreted on that basis, including the interrelations of tumor size, grade, and malignancy. The increasing age-adjusted incidence of diagnosed prostate cancer is attributed to new diagnostic tools and increased diagnostic zeal.