Hospital workers are known to be at risk of physical assault. The objective of this study is to characterize injuries resulting from physical assault among hospital nursing staff and to identify associated risk factors. Workers' compensation reports linked to human resources data were used to identify a cohort of aides and nurses employed in acute care units at a major healthcare system from 1997 to 2004 and their reported physical assault events. Poisson regression methods were used to estimate rates and rate ratios (RR) by occupation, gender, race, age, tenure, and hospital unit. During the study period 220 assaults were reported; the overall incidence rate was 1.65 (95% CI: 1.45-1.89) per 100 full-time-equivalent employees (FTEs). Assault risk was higher among those with shorter tenure (< 5 years vs. 15 or more) (RR=1.35, 95% CI: 0.83-2.19) and younger workers (under age 30 vs. 50 or older) (RR=1.30, 95% CI: 0.78-2.19), and lower among Black workers (vs. non-Blacks) (RR=0.63, 95% CI: 0.45-0.90). Incidence rates were highest in Psychiatry (12.65, 95% CI: 8.90-17.99), Neurology (4.43, 95% CI: 3.17-6.20) and Rehabilitation (3.63, 95% CI: 1.51-8.71) units. Interventions targeting Psychiatry, Neurology, and Rehabilitation units, and younger and newly hired staff are warranted. More detailed data are needed to develop targeted interventions.