We studied the between-therapist reliability and the validity of four instruments in measuring lumbar spine curvature and pelvic tilt. The four instruments and their measurements were 1) a tape measure to measure the change in lumbar curvature during trunk flexion; 2) a gravity goniometer to measure pelvic angle and lumbar curvature during stance, trunk flexion, and trunk extension; 3) a parallelogram goniometer to measure lumbar curvature during stance, trunk flexion, and trunk extension; and 4) a standard goniometer to measure the angle between wooden pointers mounted perpendicularly to the spine to obtain pelvic angle and lumbar curvature during stance, trunk flexion, and trunk extension. We found no single instrument to be the most reliable or valid. Between-therapist reliability ranged from .64 to .93 (Pearson product-moment correlation) and from .60 to .92 (interclass correlation coefficient). The validities of the instruments compared with measurements from roentgenograms generally were low, ranging from -.13 to .76 (Pearson product-moment correlation) and -.73 to -.05 (interclass correlation coefficient).