More workers' compensation claims for soft-tissue injuries are filed on Monday than on any other day of the week. Explanations for the Monday claims include "warm-up" or ergonomic effects, false classification of off-the-job weekend due to economic incentives, or psychological responses to Monday work. To sort out these possibilities, the authors examine more than 200,000 employment day patterns for a single, large U.S. employer with uniform human resource policies. Although the authors find more soft-tissue injury claims (mostly sprains and strains) filed for younger workers, union members, and for workers with higher expected workers' compensation benefits, they do not find that these factors�nor the absence of health insurance�differentially increase soft-tissue injury filings on Monday. Moreover, comparing soft-tissue injuries with lacerations and broken-bone claims suggests soft tissues are not due to ergonomic factors either. Additional evidence suggests that workers simply do not like Monday work. Hence, it may be cost-effective for employers and employees to institute practices that make the Monday workplace more attractive.