The Syrian golden hamster flank organ, composed of clusters of large sebaceous glands, has been used to investigate effects of systemically or topically applied hormones, antiandrogens, and retinoids. Accurate assessments of flank organ sebaceous gland volume and its correlation with in vivo measurements of flank organ size have not been reported. Sixty flank organs were examined. In vivo flank organ size was expressed as the product of the greatest longitudinal (DL) and greatest transverse (DT) diameters of the palpable bulk. Sebaceous gland volumes were calculated from planimetric analyses of serially sectioned specimens of flank organs using a computer-assisted planimetry system. In vivo measurements (DL X DT) were found to correlate with planimetrically determined flank organ volume. Planimetric analysis of every 10th section of a serially sectioned flank organ was almost as sensitive as analysis of every section in determining sebaceous gland volume. Computer-assisted planimetry of serially sectioned flank organs was found to be an accurate and reproducible method for assessing gland volume. In vivo measurements of the palpable bulk are a good indicator of sebaceous gland size and correlate well with planimetrically determined sebaceous gland volumes.