Forty-two neurologically intact adults in whom non-operative treatment of grade-I or grade-II isthmic spondylolisthesis of the most caudad lumbar segment had failed were entered into a prospective study of the results of operative treatment. Twenty patients who smoked were managed with a posterolateral arthrodesis with instrumentation (transpedicular fixation), and twenty-two patients who did not smoke were managed with a posterolateral arthrodesis without instrumentation. Of the patients who were managed with instrumentation, eight were randomized to treatment with a decompressive laminectomy and twelve, to treatment without it; in the group that was managed without instrumentation, the distribution was ten and twelve patients, respectively.The patients were followed clinically for a mean of 4.5 years (range, 3.5 to six years). Of the eighteen patients who had been managed with decompression, four had a pseudarthrosis and six had an unsatisfactory result compared with none and one of the twenty-four who had been managed without decompression (p = 0.02 and p = 0.01, respectively). In the group of twenty patients (smokers) who had been managed with instrumentation, none of the twelve managed without decompression had a pseudarthrosis compared with one of the eight managed with decompression (p = 0.2). In the group of twenty-two patients (non-smokers) who had been managed without instrumentation, none of the twelve managed without decompression had a pseudarthrosis compared with three of the ten managed with decompression (p = 0.04). In the group managed with instrumentation, two of the eight who had had decompression had an unsatisfactory result compared with none of the twelve who had not had decompression. In the group managed without instrumentation, four of the ten who had had decompression had an unsatisfactory result compared with one of the twelve who had not had decompression.The addition of decompression to arthrodesis, performed with or without instrumentation, for the treatment of low-grade isthmic spondylolisthesis in patients who do not have a serious neurological deficit does not appear to improve the result and may significantly increase the rates of pseudarthrosis and unsatisfactory results.