OBJECTIVE: To describe the prevalence and clinical characteristics of primary care patients who report suicidal ideation during the month before their medical visit. DESIGN: Analysis of a self-administered suicidal ideation screening item using sociodemographic data, treatment history, and clinical data from structured interviews. SETTING: Three Rhode Island private family practices, a South Carolina family medicine residency, and a California prepaid internal medicine group practice. PATIENTS: Adult primary care patients (N = 2,749), 18 to 70 years old, who are able to read and write English, able to complete study forms, and willing to provide informed consent. RESULTS: Sixty-seven (2.44%) of the patients reported suicidal ideation (''feeling suicidal'') during the past month, and most of these patients (58.2%) received no mental health care during that time. The adjusted risk of suicidal ideation was significantly elevated for patients with self-reported fair or poor physical health (odds ratio [OR] 2.5: 95% confidence interval [CI] 1.5, 4.1), fair to poor emotional health (OR 18.0; 95% CI 8.8, 37.0), marital distress (OR 4.4: 95% CI 2.2, 8.8), and recent mental health-related work loss (OR 6.3; 95% CI 3.7, 10.5). In the California sample, patients with major depression (OR 31.2; 95% CI 12.8, 76.1), generalized anxiety disorder (OR 23.4: 95% CI 8.1, 67.1), and drug abuse or dependence (OR 9.6; 95% CI 2.9, 31.6) were at increased risk of suicidal ideation. The ''feeling suicidal'' item identified 10 of 12 patients who acknowledged a recent plan to kill themselves. CONCLUSIONS: In these primary care patients, suicidal ideation is strongly associated with mental disorder and mental health-related functional impairment, and can be detected with a single self-report ''feeling suicidal'' item.