Objective To evaluate the values of whole body diffusion weighted imaging （DWI） in screening primary unknown tumor in patients with metastases. Methods Totally

34 patients with metastases of primary unknown tumors were scanned with whole body DWI

and conventional magnetic resonance （MR） imaging was performed if suspected lesions were detected. All the metastases including 27 cases of osseous metastases

2 brain metastases

2 liver metastases

1 pulmonary multiple metastasis

1 neck metastasis and 1 malignant ascites

were diagnosed by computed tomography

single photon emission computed tomography

or MR imaging. For the proven primary tumors diagnosed by biopsy or pathology of surgical specimens

apparent diffusion coefficient （ADC） values of the primary and metastatic lesions were measured respectively. The sensitivity and specificity of this technique for screening primary tumors were evaluated. Results We found 24 cases with suspected primary lesions

in which 23 lesions were proved to be primary tumors

and 1 was proved to be benign lesion. And no definite primary lesion was found in 10 cases on whole body DWI

but in which 1 case was diagnosed with primary tumor by biopsy later

and the other 9 cases remained unknown within follow-up of over half a year. The difference was not significant in ADC values between primary and metastatic lesions （P>0.05）. The sensitivity and specificity of whole body DWI for searching primary tumors was 95.8% and 90.0%

respectively. Conclusion Combined with conventional MR scanning

whole body DWI can help to search primary lesions of patients with metastases.