Although the coexistence of carpal tunnel syndrome and trigger digit in the same hand is well documented, the interactive relationship between them is unclear. This study was conducted to examine the factors that may impact on the development of trigger digit in the hand with idiopathic carpal tunnel syndrome. One hundred and eighty-one patients were enrolled. Their gender, age, dominant side, severity of carpal tunnel syndrome, history or presence of trigger digit, and treatment type were recorded. The patients were evaluated for the presence of trigger digit at 1, 3, 6 and 12 months after surgery or the initial evaluation. Two hundred and sixty-five hands in 152 patients were included in the final assessment. Logistic regression analysis revealed that surgery was a significant risk factor for the onset of trigger digit and may accelerate development of trigger digit when carpal tunnel syndrome was mild to moderate, but not when it was severe. In severe disease, other unknown factors, such as hypertrophy of the flexor tenosynovium, may mask the effect of surgery.