Shoot dieback of lodgepole pine in the British Isles has been a long standing problem, but it was not known whether or not a pathogen was involved. During 1980--1982, a shoot dieback disease of lodgepole, similar to those previously observed at different places, was studied at the Glengarry and Eilanreach forests in the northwest of Scotland. Dieback symptoms and stages in their development were investigated. A thorough review of the literature on dieback disease of lodgepole pine indicated that different disease symptoms were involved in the present outbreaks. A hitherto undescribed fungus, Ramichloridium pini de Hoog & Rahman was found to be closely associated with the early stages of discolouration of the buds and shoots of lodgepole pine while Sclerophoma pythiophila (Cda.) Hohn was most prevalent on healthy and dead tissues. A new modified Czapek Dox Agar medium has been found to suppress S. pythiophila significantly while allowing normal growth of R. pini at about 15°C, which is the optimum temperature for the latter. A method of mass production of inoculum was devised. For germination of the conidia of R. pini free water is not essential. Good germination of conidia and elongation of germ tubes occurred on undetached flushing needle sheaths and bud scales. Extensive artificial inoculations have firmly established that R. pini is the primary pathogen of the present shoot dieback disease of lodgepole pine. Most successful infections were obtained from inoculations carried out in May and June. By October, shoots became resistant to infection. Plants of the Central Nass River provenance were more resistant to R. pini than those from Long Beach, Washington. It has been shown that conidia produced on needle fascicle scars could provide natural inoculum. The infection period lay between mid April to mid May or up to early June at Eilanreach in 1981.