Typical asthmatic reactions after exposure to common or occupational allergens have been classified as isolated immediate, early late, late, and dual (Pepys and Hutchcroft, 1975). Atypical reactions can also occur, but their temporal behavior and frequency are unknown. We analyzed 69 bronchospastic reactions after exposure to three types of occupational sensitizers in the laboratory: isocyanates, western red cedar, and IgE sensitizing agents. Graphs of the reactions were presented to three observers in a blind, randomized way. Reactions were defined as follows: (1) typical patterns, that is, isolated immediate, early late, late, and dual, as put forward by Pepys and Hutchcroft, and (2) atypical patterns including progressive (onset, during, or minutes after exposure, progressing to a maximum reaction 5 to 6 hours later) and square waved (similar to a dual reaction but with only partial recovery [less than 10%] between the immediate and late falls in FEV1). Some curves were also analyzed by polynomial regression. The three observers agreed in 59/69 cases (86%), and a consensus was reached after discussion for the remaining subjects. Fifteen of 63 (22%) reactions were atypical. IgE-mediated reactions and reactions to red cedar were, respectively, mainly of the immediate and late types. The distribution of reactions to isocyanates, however, was significantly different (chi-square, 6.1; p = 0.01), the "progressive" pattern occurring in 7/23 instances (30%). There was a satisfactory concordance between visual assessment and polynomial fit analysis in distinguishing dual from square-waved reactions. We conclude that isocyanates frequently cause atypical progressive bronchospastic reactions that are rarely observed after exposure to IgE agents and western red cedar.