Pathological classification and follow-up of prostatic lesions initially diagnosed as "suspicious of malignancy". Sections from a series of prostatic specimens (n = 44) in which the initial diagnosis implied a suspicion of malignancy were reviewed and the patients followed up (group 1). The aim was to categorise the morphological appearances according to current criteria and to determine the natural history of any pre-malignant lesions within this group. A "control" series of patients (n = 40) in whom the primary diagnosis was benign were also reviewed and their course followed (group 2). The heterogeneity of the abnormalities in group 1 was striking. Of the 44 cases, pre-malignant lesions were seen in 8, microinvasive adenocarcinoma in 11 and lesions not now considered pre-malignant in the remaining 25. None of the 8 patients with pre-malignant lesions developed clinical evidence of carcinoma over a follow-up period of 8 to 12 years. In contrast, 3 of the 11 diagnosed on review as having microscopic invasive adenocarcinoma developed clinically evident tumour within 5 years of surgery. Of the 25 patients whose prostatic lesions were considered benign, 2 developed carcinoma 12 and 13 years after surgery. Only 1 of the 40 patients in group 2 had a pre-malignant lesion and he did not develop tumour within 10 years. However, adenocarcinoma was diagnosed incidentally in 1 patient 3 years after surgery. It was concluded that there is a need to standardise interpretation and reporting of pre-malignant lesions and microscopic foci of adenocarcinoma in the prostate. The limited results on the course of pre-malignant lesions diagnosed incidentally in patients over 60 years of age do not indicate that follow-up would have prompted the early diagnosis of invasive disease. 