Anatomical variations of the first dorsal fibroosseous compartment were studied in 70 patients (38 women, 32 men) who underwent surgical release of the tendons. Seventy-one dissected forearms were also studied. De Quervain's tenosynovitis was the only diagnosis for two thirds of the patients. Nonoperative treatment for all of these patients failed. Forty-three patients (61%) had job-related symptoms. Recovery ranged from 3 to 36 months postoperatively (mean: 4.5 months). Multiple subcompartments were recorded in 47% of patients and in 75% of anatomic specimens. In 94% of the specimens, the abductor pollicis longus (APL) tendon had two to four slips, but in only two were there multiple extensor pollicis brevis (EPB) tendons. Surgeons must recognize that multiple APL tendon slips and two subcompartments are the rule, rather than the exception, in normal anatomy.