Despite widespread clinical and epidemiologic attention to adverse drug reactions (ADRs), their clinical identification has been a nonreproducible act of unspecified subjective judgment; adequate operational criteria have not been available for diagnostic decisions about the cause of an observed untoward clinical manifestation. To improve scientific precision in the diagnosis of ADRs, we have developed an algorithm that provides detailed operational criteria for ranking the probability of causation when ADR is suspected between a drug and a clinical manifestation. The algorithm provides a scoring system for six axes of decision strategy: previous general experience with the drug, alternative etiologic candidates, timing of events, drug levels and evidence of overdose, dechallenge, and rechallenge. The sum of the scores is ordinally partitioned to rate the candidate ADR as definite, probable, possible, or unlikely.