A prospective study was undertaken of fifty-one patients who were randomly assigned to treatment with either a long or a short thumb-spica cast for a non-displaced fracture of the carpal scaphoid. The duration of follow-up was at least until union; the average follow-up was twelve months. Twenty-eight fractures were treated with a long thumb-spica cast and twenty-three, with a short thumb-spica cast. The hands that initially were treated with a long thumb-spica cast were placed in a short thumb-spica cast after six weeks. Fractures that initially were treated with a long thumb-spica cast united at an average of 9.5 weeks and those that were maintained in a short thumb-spica cast, at an average of 12.7 weeks. There were no non-unions and two delayed unions in the fractures that initially were treated with a long thumb-spica cast, compared with two non-unions and six delayed unions in those that had only a short thumb-spica cast. Fractures of the proximal or middle third of the carpal scaphoid had a significantly shorter time to union when they were treated initially in a long thumb-spica cast. Fractures of the distal third did well regardless of the type of immobilization.(ABSTRACT TRUNCATED AT 250 WORDS)