This cohort study is among the first to estimate the prevalence of and examine potential risk factors for severe back pain (resulting in medical care, 4+ hours of time lost, or pain lasting 1+ weeks) among adolescent farmworkers. These youth often perform tasks requiring bent/stooped postures and heavy lifting. Of 2536 students who participated (response rate across the three public high schools, 61.2% to 83.9%), 410 students were farmworkers (largely Hispanic and migrant). Students completed a self-administered Web-based survey including farm work/nonfarm work and back-pain items relating to a 9-month period. The prevalence of severe back pain was 15.7% among farmworkers and 12.4% among nonworkers. The prevalence increased to 19.1% among farm workers (n = 131) who also did nonfarm work. A multiple logistic regression for farmworkers showed that significantly increased adjusted odds ratios for severe back pain were female sex (4.59); prior accident/back injury (9.04); feeling tense, stressed, or anxious sometimes/often (4.11); lifting/carrying heavy objects not at work (2.98); current tobacco use (2.79); 6+ years involved in migrant farm work (5.02); working with/around knives (3.87); and working on corn crops (3.40). Areas for further research include ergonomic exposure assessments and examining the effects of doing farm work and nonfarm work simultaneously.