In August 2003, an outbreak of scombroid fish poisoning occurred at a retreat centre in California, USA. In a retrospective cohort study, 42 (75%) of the 56 dinner attendees who ate escolar fish (Lepidocybium flavobrunneum) met the case definition. Individuals who ate at least 2 oz of fish were 1·5 times more likely to develop symptoms than those who ate less (relative risk 1·5, 95% confidence interval 0·9–2·6), and to develop more symptoms (median 7 vs. 3 symptoms, P=0·03). Patients who took medicine had a longer duration of symptoms than those who did not (median 4 vs. 1·5 h, P=0·05), and experienced a greater number of symptoms (median 8 vs. 3 symptoms, P=0·0002). Samples of fish contained markedly elevated histamine levels (from 2000 to 3800 ppm). This is one of the largest reported outbreaks of scombroid fish poisoning in the United States and was associated with a rare vehicle for scombroid fish poisoning, escolar.