Posterior lumbar interbody fusion (PLIF), as recommended with bilateral lumbar interbody cages and pedicle screw fixation, has increased the successful fusion rate to nearly 100%. Presently, a unilateral approach to the disc space with a variant of PLIF, the trans-foraminal interbody fusion is often used. There are few clinical studies of unilateral interbody fusion. The clinical and fusion results of unilateral interbody fusion are important as the usage of trans-foraminal interbody fusion procedure increases. This retrospective study of 26 consecutive patients treated with a unilateral cage asks whether fusion healing and clinical outcome is comparable with that obtained with bilateral cages. In this study, there were no pseudarthroses, instrumentation failures, or significant subsidence at any of the single cage levels. Disc space height and foraminal height were restored by the surgery and maintained at last follow-up. Using Prolo scores, 23/26 patients had clinical success (88%), and 3 were unsuccessful. Fusion was successful at all single cage fusion levels and overall in 23/26 (88%) reviewing all levels of fusion. In conclusion, fusion and clinical success rates were not diminished by the use of a unilateral interbody cage rather than the recommended 2 cages. This retrospective comparative study is a Level III-2 Therapeutic Study investigating the results of unilateral PLIF with a single interbody cage compared with historical series with interbody cages.