To test the construct validity of postural backache. To identify if individuals with backache sit for longer periods of sustained sitting and have more flexed relaxed sitting posture than individuals in a no backache group.Following an initial questionnaire, student volunteers without a history of 'serious' back pain were classified as either postural backache group or no backache group. With the use of an activity diary that plotted activity every 5 minutes over a 3-day period, the average time spent in different postures was established. Relaxed sitting posture was evaluated using Dartfish software to analyze videotape after 10 minutes of sitting.The most common daytime activity of both groups was sitting, with average sitting time not significantly different between groups. Periods of uninterrupted sustained sitting and uninterrupted sustained studying were significantly longer, and the degree of flexion in relaxed sitting was significantly greater in the postural backache group (all P < .024).In a group of student volunteers, half reported postural backache. The group with backache sat for longer periods without interruption and had a more flexed relaxed sitting posture than the no backache group. These findings appear to validate McKenzie's concept of a postural syndrome.