Epicutaneous skin testing is a useful diagnostic tool in evaluating allergic disorders. Utilizing the Multitest device, skin prick test responses to codeine phosphate, histamine phosphate, and ragweed were examined in 56 human subjects. Relationships between the two positive controls, codeine and histamine, and their use as a reference denominator for ragweed reactions were assessed. Ragweed elicited detectable wheals in 15/56. Histamine phosphate (2.75 mg/mL) elicited a positive wheal response in 52/56 subjects, while codeine phosphate elicited a positive wheal in 39/56 and 30/56 subjects at 30 and 3 mg/mL, respectively. Wheal sizes for codeine phosphate at both 30 and 3 mg/mL showed significantly concordant relationships with histamine phosphate-induced wheal sizes (Spearman rho, P = .0084 and .0155, respectively); however the intersubject coefficient of variation was lower for histamine-induced wheal sizes (44%) than for codeine-induced wheal sizes (64% and 65%, respectively for 30 and 3 mg/mL). When a ratio of allergen to positive control reaction size was used to grade ragweed reactions, different patterns were observed using codeine compared with histamine. These results have implications in utilizing codeine phosphate as a positive skin prick test control for allergy testing.