The records of 98 patients with 160 hypoplastic thumbs seen between 1923 and 1993 were reviewed to determine the salient characteristics of this population. Physical findings, photographs, and x-ray films were studied. Sixty-three percent of patients were male, and 63% had bilateral thumb hypoplasia, 59% had radial dysplasia, and 86% had other anomalies. An associated syndrome, most commonly the vertebral, anal, tracheosophageal, renal, and radial limb anomalies association or Holt-Oram syndrome, was present in 44%. Patients with spine, genitourinary, or gastrointestinal anomalies were most likely to have the vertebral, anal, tracheosophageal, renal, and radial limb anomalies association, those with cardiac anomalies were most likely to have Holt-Oram syndrome, and those with lower extremity anomalies were most likely to have a different syndrome. One hundred thirty-nine thumbs were classifiable, using a modification of Blauth's classification; 19% were types 1 and 2, 23% were type 3, and 58% were types 4 and 5. One hundred seven operations were performed on 63 upper limbs, including 24 thumb reconstructions and 35 pollicizations. These patients must be carefully examined for bilaterality, other anomalies, and syndromes. Classification using a modification of Blauth's criteria is useful and helps the surgeon determine a treatment plan.