Previously published epidemiologic studies of low back pain (LBP) have reported that the prevalence of low back disability has increased dramatically. These studies based their findings on either the number of disability claims filed, the disability duration, or both. This information was from countries other than the United States or from the US Social Security Disability Insurance data, with findings reported only to the early 1980s. More recent studies of US workers' compensation LBP claims reported a decrease in the incidence rate from the late 1980s to the mid-1990s. No studies have been found that report on the trends of disability duration for workers' compensation LBP claims. This study examined recent trends in the length of disability (LOD) for LBP claims and associated costs, using a large sample of claims from the privately insured US workers' compensation market. LOD and cost information were derived for injuries from 1988, 1990, 1992, 1994, and 1996. For each year, the distributions of LOD and cost were skewed, with the small percentage of claims that lasted more than one year (4.6%-8.8%, depending on the year) accounting for a large percentage of the total disability days (77.6%-90.1%) and cost (64.9%-84.7%). From 1988 to 1996, the average LOD decreased 60.9%, from 156 days to 61 days. The probability of being on disability for a long period of time has decreased over the years. Over the study period, the average cost of a claim decreased 41.4%, while the median cost increased 19.7%. The most influential change in the LOD and cost distributions was a reduction in expensive claims with a long disability duration. The evolution of LOD and cost is also detailed for different disability durations for the study period.