All, I have a relatively new EOS 40D body. At the time the camera was purchased, I also purchased a Sgima 100-300 mm/f4 DG lens. The lens functioned properly for a period of time (not more than several hundred images), at which point the autofocus functionality stopped working. When I mounted my other lens, a Sigma 18-50 mm/f2.8 lens, it functioned properly, leading me to the conclusion that the problem is with the lens. Since the lens was covered by warrantee, I returned it to Sigma. Upon their examination, they claimed the problem was a failed HSM motor, which they replaced. When I received the lens, I mounted it to the camera, snapped a few pictures, and the lens appeared to work. Several week later, I again mounted the lens, but it failed in the exact same manner. I spoke with Sigma and they agreed to again repair the lens (and did so quickly since I was about to leave for a trip). This time, they replaced both the HSM and the electronics control module. As you may have guessed, the lens again failed, in the exact same manner, during the course of my travels. Sigma has again agreed to repair the lens, but I am concerned that they are missing something. Here is what is known: The lens will work (after repair) for a period of time, then fail. The failure mode is that the autofocus stops functioning. However, it still works in manual focus mode. My other lens, also made by Sigma, continues to operate properly. I mounted the problematic lens on another Canon body (EOS 50D at a camera shop) and the lens would not autofocus. Sigma claims that when they received the lens the second time, they mounted it on a camera body and it functioned properly. I have tried mounting the lens numerous times, but this did not resolve the problem. Per Sigma’s recommendation, I exercised the manual focus, but this did not resolve the problem. The problem occurs even with a freshly charged battery. Before I again return the lens to Sigma, can you suggest any other tests I should try or any specific instructions for Sigma? Thank you.