An electronic bone growth stimulator (EBGS) had been devised to facilitate lumbosacral fusion in the early 1970s, using a totally implanted device which delivers a steady 5 microA at each of four cathodes. The use of the device is reserved for patients in whom painful and abnormal motion is demonstrated to be the major cause of low-back pain. An initial series (I-A) of 82 patients demonstrated a successful fusion rate of 91.5% compared with a 80.5% fusion rate in 159 patients treated without the EBGS. A second prospective randomized controlled trial series (II) was begun using only "difficult patients," that is, patients who had either one or more previous failed fusions, a grade II or worse spondylolisthesis, a multiple-level fusion or the presence of another high-risk factor such as obesity. These patients were randomized by protocol as to whether they received a stimulator or not. The fusion rates of the two groups were assessed radiographically by the operating surgeon and by an independent radiologist 12 to 18 months postoperatively. Statistical review confirmed the comparability of the groups. Success, defined as radiographic fusion, was achieved in 15 of 28 control patients (54%) compared with 25 of 31 EBGS treated patients (81%). This result is statistically significant (P = 0.026, one-tailed Fisher's Exact test). Meanwhile, a continuing nonrandomized study (I-B) has continued and at this point the success rate is 90.5% in an additional 116 patients, confirming the results of the earlier nonrandomized series (I-A).(ABSTRACT TRUNCATED AT 250 WORDS)