Twenty-five out of 41 adult patients suffering from their first attack of acute low-back pain were subjected to local anesthetic injection into the maximally tender lumbar apophyseal joints under controlled conditions. Two clinical syndromes can be identified according to whether the patients responded to the facet injection or not. Various clinical differences were statistically significant and included a wider than normal spinal canal in the responding group. The clinical significance of these two distinct groups is discussed in detail.