In a prospective study of risk factors for lateral ankle sprain among 390 male Israeli infantry recruits, a 18% incidence of lateral ankle sprains was found in basic training. There was no statistically significant difference in the incidence of lateral ankle sprains between recruits who trained in modified basketball shoes or standard lightweight infantry boots. By multivariate stepwise logistic regression a statistically significant relationship was found between body weight x height (a magnitude which is proportional to the mass moment of inertia of the body around a horizontal axis through the ankle), a previous history of ankle sprain, and the incidence of lateral ankle sprains. Recruits who were taller and heavier and thus had larger mass moments of inertia (P = 0.004), and those with a prior history of ankle sprain (P = 0.01) had higher lateral ankle sprain morbidity in basic training.