A vesicular, peeling rash characteristic of a phytophototoxic dermatitis developed on the hands and arms of 30 of 127 grocery workers. The rash subsequently healed with residual hyperpigmentation. Produce workers had the highest attack rate, 100% (8 of 8, p less than 0.0001). Although contact with celery had the highest relative risk for disease (relative risk, 6.3; 95% confidence interval, 2.6, 19.2) and the strongest dose-response effect, an association with celery alone could not be shown because workers were also exposed to other produce. However, anecdotal evidence also suggested that celery might be involved. The disease-resistant, high-quality brand of celery carried by these stores had higher levels of furanocoumarins, potent photosensitizers and a known cause of phytophotodermatitis, than other brands (p = 0.01). A randomly selected nationwide sample of stores in this chain showed dermatitis in 13 of 17 states and 26% of produce workers surveyed. Plant breeding to produce a more disease-resistant celery stock may lead to increased levels of endogenous furanocoumarins, resulting in phytophotodermatitis in grocery workers.