Background: Emerging targeted and combination treatments of novel anticancer drugs in the molecular era mandate an increasing number of paired fresh tumour biopsies. We aimed to assess risks and complications of Ultrasound  US  guided biopsies performed in patients  pts  enrolled in phase I trials. Methods: Retrospective analysis of 350 cases of biopsies performed ithin the Drug Development Unit  DDU  at the Royal Marsden Hospital. Data ere collected from electronic pt notes beteen August 2008 and December 2015. Results: Three hundred and forty  340  biopsies ere performed on 195 pts participating in 46 different phase I trials. Nine biopsies ere not performed due to medical reasons and one patient declined. 139 pts  72 percent  had paired biopsies, 13 pts had more than 2, and 3 pts had paired biopsies in the same day. Of all biopsies 287  82 percent  ere mandatory and 177  51 percent  ere superficial. The most commonly biopsied organ as the liver  137 . All biopsies ere performed under US guidance by one of the to DDU-dedicated radiologists. The median age as 59  27-79  ith 60 pts being older than 65y. Median ECOG performance status as 1  0-3  and predominant gender as male  100195 . 24  12 percent  pts ere on anti-coagulation treatments and 9 had thrombocytopenia National Cancer Institute Common Toxicity Criteria  NCI-CTC  Grade 1 or 2. All biopsies ere core biopsies; the number of cores ranged beteen 2-6. The most commonly needle size used as 18G. Reported complications included post procedure pain 16  4.7 percent , infection 2  0.6 percent  and conservatively treated bleeding 2  0.6 percent , all at grade 1. . None of the complications required admission to hospital or prolongation of admission. Conclusions: In the era of ne molecular targeted therapies and combination strategies, biopsies are necessary for the comprehension and further development of such treatments. We have demonstrated that paired, fresh biopsies in the context of phase 1 trials, can be performed safely using US guidance ith minimal complication rates in all age groups.,J Clin Oncol 34, 2016  suppl; abstr 11579 00:00.0,Tumor Biology