47 patients with spongiform leucoencephalopathy but no other consistent abnormalities, except brown pigmentation of the alveolar macrophages in the lungs, are described. 11 patients have died. Epidemiological studies indicate at the cause of the illness the inhalatory use of poisoned heroin vapours (pyrolysate). The heroin is primarily sold on the black market in Amsterdam. The Netherlands. Over 170 suspect heroin samples were collected for analysis of the possible poisonous factor. Although suspect, none of the samples could be unambiguously related to the observed illness. Chemical, toxicological, and histopathological investigations have not so far revealed the nature of the poisonous factor, but several neurotoxic agents that are known to cause comparable leucoencephalopathies have been ruled out. This appears to be the first manifestation of the poisonous potential of the unknown causative factor.