The helicopter was being used in an emerg med svc/med-evac (EMS) opn. At 0230, the plt obtained a partial wx briefing. He was advised of flt prctns for IFR wx thru-out the area with low ceilings, fog, precip & mtns obscd. The flt departed Nashville at 0245 & lndd at Jamestown at 0333. About 17 min later, it took off on the return flt to Nashville with 2 medpsnl & a patient aboard. About 3.2 mi fm the departure point, the acft hit trees moments after crossing hi terrain along the rim of a valley. The impact area was void of lgts. A wx study indcd cld bases at about 800' AGL, vis less than 3 mi in drizzle & fog. Psnl in the area estd 200' to 300' overcast, vis 1/2 to 3/4 mi. No preimpact part failure/malfunction was fnd. The EMS opn was certified under 14 CFR 135 (no specific reg for EMS opns). The company opns manual lacked guid-ance concerning EMS opns. The operator did not have an FAA approved training manual. No FAA record was fnd showing a base inspection by either opns or maint inspectors btn 1/11/85 & the date of the acdnt.