Steady increases in the costs of medical care, coupled with a rise in the fraction of workers who lack medical care insurance, have led to a growing concern that the Workers' Compensation system is paying for off-the-job injuries. Many analysts have interpreted the high rate of Monday injuries -- especially for hard-to-monitor injuries like back sprains -- as evidence of this phenomenon. In this paper, we propose a test of the hypothesis that higher Monday injury rates are due to fraudulent claims. Specifically, we compare the daily injury patterns for workers who are more and less likely to have medical insurance coverage, and the corresponding differences in the fraction of injury claims that are disputed by employers. Contrary to expectations, we find that workers without medical coverage are no more likely to report a Monday injury than other workers. Similarly, employers are no more likely to challenge a Monday injury claim -- even for workers who lack medical insurance.