Frequency distributions and incidence ratios pertaining to the incidence of workers' compensation claims due to strain/sprains of the back were determine by analysis of data obtained in 1979 in 26 states by the Bureau of Labor Statistics' Supplementary Data System (SDS). In 1979 injuries to the back accounted for almost 20 percent (329,474) of all compensation claims, and strains/sprains accounted for 86.6 percent of all back compensation claims. Males produced 76 percent of all claims, but the average ages for males (33.8 years) and females (34.6 years) filing back compensation claims were nearly identical. The largest risk of compensable low back strain/sprain was calculated to occur between the ages of 20 to 44 years. The male to female risk ratio ranged from 1.4 in the group 55 to 64 years old to 3.0 in the group 20 to 24 years old. Lifting of objects produced almost half (48.1 percent) of the back strains/sprains, followed by pulling or pushing (9.0 percent), miscellaneous overexertions (9.0 percent), and voluntary body motions (6.6 percent). Manufacturing had the largest number of claims, followed by trades and services. Adjusting the number of claims to the number of workers employed showed that the construction industry had the largest ratio of claims for back injuries (1.6 per 100 employees), followed by the mining industry (1.5), the transportation industry (1.2) and the manufacturing industry (1.0). The frequency of back strains/sprains by occupation showed that miscellaneous laborers had the highest ratio of claims per 100 workers (12.3) followed by garbage collectors (11.1), and warehousemen (9.3). Lifting boxes accounted for the largest number of claims (9.3 percent). The average direct cost, medical payment plus indemnity compensation, for a strain/sprain of the low back was 3,533 dollars. The authors conclude that the data suggest that back injuries continue to be a large and costly problem for U.S. workers and their employers.