We detected 19 cases of phytophotodermatitis during a cross-sectional epidemiological investigation of two Oregon grocery stores that were part of the same supermarket chain. Outdoor sunlight exposure during the workshift and tanning salon use were identified as risk factors; the most severe cutaneous reactions tended to occur among tanning salon users. Although both stores carried the same brands and varieties of produce, all 19 cases occurred among employees of one store, which had held a celery sale coincident with the outbreak, resulting in a quadrupling of the usual volume of celery sold. We found elevated psoralen levels in two of three celery samples obtained from the affected store; cutaneous provocation tests with trimmed surfaces of these celery samples produced phototoxic reactions. Preliminary experiments with one brand of celery have demonstrated psoralen levels as high as 25 micrograms/cm2 of trimmed surface. These observations suggest that clinical phytophotodermatitis among grocery store workers may be caused by healthy celery and results from a complex interaction of exposure variables, including ultraviolet radiation from tanning salon use, frequency of handling celery, celery brand, and sporadic elevation of psoralen content from environmental stresses.