On April 8, 2009, the Los Angeles Police Department (LAPD) notified officials from the Los Angeles County Department of Public Health (DPH) in California about a group of preschool teachers with nausea, dizziness, headache, and numbness and tingling of fingertips after consumption of brownies purchased 3 days before from a sidewalk vendor. To characterize the neurologic symptoms and determine whether these symptoms were associated with ingestion of the brownies, the police and health departments launched a collaborative investigation. This report summarizes the results of that investigation, which detected cannabinoids in a recovered sample of the brownies. Two patients sought medical attention, and one patient's urine and serum tested positive for 11-nor-9-carboxy-delta 9-tetrahydrocannabinol (THC-COOH), a marijuana metabolite. The findings in this report demonstrate the utility of a collaborative investigation by public health and law enforcement.The findings also underscore the need to consider marijuana as a potential contaminant during foodborne illness investigations and the importance of identifying drug metabolites by testing of clinical specimens soon after symptom onset.