A panel of toxicology and medical experts was convened on 7-9 April 1997 to consider the available scientific literature on chlorpyrifos, both published and unpublished, to determine the acute and chronic toxicology reference dose (RfD). In the course of reviewing this literature it became apparent that there was a large body of literature on human exposures to chlorpyrifos, as well as chlorpyrifos and/or other organophosphates. This literature, although not useful for determining the RfD for chlorpyrifos, needed to be analyzed for potential critical human effects resulting from either acute or prolonged chlorpyrifos exposures, or inferred from exposures to other organophosphates. The expert panel proceeded to review these data also, and the evaluations and discussions of these studies are contained in this report of the proceedings. The expert panel concluded that for acute poisonings there was no clear evidence for long-term effects from organophosphates, other than finding cases of organophosphorus-induced delayed neurotoxicity (OPIDN) from suicidal ingestion. In animal experimental data (mainly from studies on nerve gases), seizures during acute poisoning by organophosphates occur, resulting in morphological damage. Neurobehavioral effects observed are the result of the seizures. The panel agreed that long-term exposure to organophosphate compounds does not cause problems in the peripheral or central nervous system, unless poisoning is acute and severe. With respect to neurobehavioral effects, manifestations of clinical neurobehavioral effects are unlikely. All of the available evidence shows that disturbances do not occur unless cholinesterase inhibition has been clearly exhibited. The review of these papers was considered to be of interest in allaying some of the potential concerns regarding long-term effects of organophosphate pesticides, including chlorpyrifos.