To ascertain the predictors of functional outcome in elderly patients undergoing posterior lumbar spinal decompression and fusion, a modified low back outcome score questionnaire survey in 83 elderly patients (49 men, 34 women) was carried out at our hospital. The average follow-up was 35.8 months (range: 22–57 months). The outcomes were as follows: excellent to good, 83%; fair, 7%; and poor, 10%. Multiple regression analysis revealed that significant predictors of unfavorable outcome included coexistence of other bone and joint degenerative disorders (p < 0.001) and history of heart disease (p < 0.01). Patients who had undergone previous lumbar surgery had significantly lower modified low back outcome score than those without previous lumbar spine surgery (p < 0.05). The modified low back outcome score questionnaire system is a reliable method in elderly patients.