Protrusio acetabuli [center-edge (C/E) angle > 60 degrees, teardrop collapsed, and pelvis invaded by acetabulum beyond ilioischial line (Köhler's)] was identified in 22 patients with classic Marfan syndrome. All had a preponderance of the syndrome's cardinal features (positive pedigree and musculoskeletal, ocular, and cardiovascular manifestations). Fifty-two immature patients with the syndrome's skeletal expressions predominating were studied for deep acetabuli. Half of these individuals (26 patients) had true protrusio acetabuli. Thirty-one percent (16 patients) had abnormal acetabular deepening (C/E angle > 50 degrees; teardrop partially collapsed without acetabular protrusion); 19% (10 patients) had normal pelves with C/E angle < 40 degrees and normal teardrop. To arrest the protrusio, the triradiate physis was closed surgically in 11 patients (21 hips), one unilaterally. In 19 operated-on hips followed-up to maturity by using teardrop, C/E angle, and acetabular relation to the ilioischial line as criteria, architecture was restored to normal in 12 hips and reduced from true protrusio to simply acetabular deepening in four. Three were unchanged. The youngest child operated on was 8; the oldest was 12 years. Laboratory examination of the physes was unrewarding.