The incidence of slipped capital femoral epiphysis (SCFE) remains controversial. A population-based database was used to identify all residents of a Midwestern American county treated for a new diagnosis of SCFE. Between 1965 and 2005, 49 patients (aged 9-16 years) underwent treatment of SCFE. This represents an annual incidence of 8.3 unilateral cases and 0.5 bilateral cases per 100,000 children. In patients with unilateral disease, mild slips developed in nine contralateral hips (19%) at a mean of 166 days (range: 6-432 days). As all contralateral slips were mild, we recommend careful follow-up rather than prophylactic pinning of the contralateral hip.