Acute bacterial prostatitis (ABP) (NIH Category I), has not undergone any modification in the update of prostatitis classification. ABP was diagnosed in 614 patients in our centre over 9 years (1993-2001). We analyse the clinical pattern of ABP and the role of bladder outlet obstruction in its etiology, as well as whether two different ABP sub-categories could be defined as a function of a history of previous manipulation of the lower urinary tract. The results of the study show that the clinical pattern of a patient suffering from ABP does not differ from the statements of previous publications. On the other hand, patients with ABP have been shown to present with no bladder outlet obstruction. Finally, this study has disclosed the fact that the cases of ABP elicited by previous manipulation of the lower urinary tract (10%) show a different pattern from those cases where no previous manipulation has occurred (90%). The patients with ABP secondary to manipulation are older, have a higher risk of prostate abscess and higher frequency of multiple infections and also infections by pathogens other than Escherichia coli. Due to all of these reasons, it would be advisable to subdivide category I within the classification of prostatitis.