Background Few research instruments used in occupational stress research have been evaluated for acceptability and validity among immigrant Latino farmworkers. Methods Cognitive testing was completed with 40 migrant and seasonal farmworkers (21 women, 19 men) through two focus groups and 16 one-on-one interviews conducted in Texas and Florida. Participants responded to the K-6, a short form instrument designed to measure psychological distress, selected items from the Job Content Questionnaire (JCQ) and standard health items. Results The K-6 items were characterized as too long and using a higher ‘‘class’’ language than farmworkers use. Further, the cultural connotation of several items in the K-6 was viewed as inappropriate by farmworkers. Demand items from the JCQ were interpreted inconsistently, whereas decision latitude items were consistently understood but viewed as irrelevant to farmworkers. Conclusions The results highlight the difficulties involved in conducting research with immigrant farmworkers, and they suggest that researchers interested in studying antecedents and consequences of farmworker mental health need to select instruments cautiously.