We performed a retrospective study of the long-term results of posterior instrumentation and arthrodesis of the spine in forty-three patients who had idiopathic scoliosis and a Risser grade of 0 at the time of the operation. The average age of the patients was 12.4 years (range, 6.7 to 15.5 years) at the time of the operation. The triradiate cartilages were open in twenty-three patients and closed in twenty. At the time of the latest follow-up evaluation (average duration of follow-up, four years; range, two to eleven years), seventeen patients had a Risser grade of 5; twenty-two, 4; two, 3; one, 2; and one, 0. The crankshaft phenomenon, a progressive deformity resulting from continued growth of the anterior aspect of the spine after posterior arthrodesis, was seen in only one patient who had closed triradiate cartilages and in ten patients who had open triradiate cartilages (p = 0.004). The most common radiographic finding was a progressive rib-vertebra angle difference, which increased more than 10 degrees in seven of the eleven patients who had the crankshaft phenomenon. The mean increase in these eleven patients was 22 degrees, compared with no increase in the thirty-two other patients (p < 0.0001). Open triradiate cartilages (r = 0.58, p = 0.0001) and a younger age at the time of the operation (p < 0.0001) were predictive of the amount of progression as a result of the crankshaft phenomenon. In patients who had open triradiate cartilages, less skeletal maturity was also predictive of progression as a result of the crankshaft phenomenon (r = -0.72, p = 0.0002).