The time patients spend in a doctor’s waiting room prior to a scheduled appointment is an important component of the quality of the overall health care experience. We analyzed data on twenty-one million outpatient visits obtained from electronic health record systems, which allowed us to measure time spent in the waiting room beyond the scheduled appointment time. Median wait time was a little more than four minutes. Almost one-fifth of visits had waits longer than twenty minutes, and 10 percent were more than thirty minutes. Waits were shorter for early-morning appointments, for younger patients, and at larger practices. Median wait time was 4.1 minutes for privately insured patients and 4.6 minutes for Medicaid patients. After adjustment for patient and appointment characteristics, Medicaid patients were 20 percent more likely than the privately insured patients to wait longer than twenty minutes, with most of this disparity explained by differences in practices and providers they saw. Wait times for Medicaid patients relative to privately insured patients were longer in states with relatively lower Medicaid reimbursement rates. The study complements other work that suggests that Medicaid patients face some additional barriers in the receipt of care.