Abstract  Inhibitor of DNA binding-1 (Id1) is a dominant-negative regulator of basic helix–loop–helix transcription factor, which control malignant cell behaviors in several types of carcinomas. This study aimed to find the relationship between Id1 expression and some clinical parameters. Paraffin-embedded tissue specimens from two normal human prostates, 12 benign prostatic hyperplasia (BPH), 43 prostate cancers(PCa) were detected by immunofluorescence assay. Prostatectomy samples from 11 BPH and 28 PCa were used for real time RT-PCR. The relationship between Id1 staining and several patient’s clinical parameters, including Gleason grade, PSA, clinical stage, and size of tumor, was further analyzed. Significant up-regulated Id1 protein was shown in prostate cancer specimens, while only weak expression in some BPH samples (5/12). Analyzed by image software, the mean proportion of Id1 positive staining remarkably increased with the increasing of Gleason grade in prostate cancer specimens (r = 0.9967, P < 0.01). Id1 expression was not significantly associated with PSA, TNM stage or tumor size. Furthermore, the average mRNA of prostate cancer was 3.09 times of BPH. This study confirms that Id1 protein and mRNA are over expressed in prostate cancer tissues. Overexpression of Id1 protein correlates with tumor tissue differentiation. We propose that Id1 over expression can be used in the analysis of the progression of prostate cancer.