Objective To evaluate the availability and sensitivity of MR whole body diffusion weighted imaging (DWI) in searching primary tumors in patients with metastases.Methods Thirty-four patients with symptoms of metastases and unknown primary leasions were scanned by whole body DWI.All the metastases cases were diagnosed by CT,single photon emission computed tomography(SPECT) or MRL For the cases of suspected primary tumors,apparent diffusion coefficient (ADC) values of the primary and metastatic lesions were measured respectively.For the cases with primary lung cancers and metastases proved by pathology of biopsy or surgical specimens,statistics analysis of ADC values of primary and metastatic lesions was performed with paired samples t-test.The detection rate and sensitivity of this technique for screening primary tumors were evaluated.Results By whole body DWI,24 cases were found with suspected primary lesions,in which 23 cases were proved primary tumors,and 1 case was proved benign lesion.Ten cases were not identified with primary lesions on whole body DWI,but in which 2 cases were proved primary malignant tumor by biopsy later,and the other 8 cases remained unknown within follow-up of half a year to one year.For the 14 definite cases of lung cancers,the average ADC values of primary and metastatic lesions was (1.24±0.18)×10-3 and(1.16±0.27)×10-3mm2/s,respectively (t=1.28,P>0.05).The detection rate and sensitivity of whole body DWI for searching primary tumors was 67.6% (23/34) and 92.0% (23/25),respectively.Conclusion The sensitivity of whole body DWI for searching primary tumors is relatively high,and it can be used to search primary tumors in patients with metastases.