Laboratory-based surveillance detected an outbreak of Salmonella Thompson infections in southern California. There were 41 cases associated with eating at a single restaurant in Los Angeles County: eating dishes that included fresh cilantro increased the risk of illness. Thirty-five additional cases were detected independent of this restaurant outbreak. Some of the cases in this group were linked to eating at two other restaurants where salsa that contained cilantro was served. Additional cases in this group had eaten cilantro at two more restaurants where salsa was not on the menu. A traceback of the cilantro was initiated but was hampered by poor record keeping.