Quantitative analysis of transmitted cardiac motion in fetal lung is evaluated by applying correlation techniques to digitized M-mode images in 21 patients, subdivided into two subgroups by gestational age: (I) 25-30 weeks (11 patients), and (II) greater than or equal to 35 weeks (10 patients). The corresponding numbers of M-mode images analyzed for each group are 23 and 18, respectively. This partition is expected to reflect functionally "immature" and "mature" lungs. The estimated maximum mean radial deformation per unit epicardial excursion, (r), is calculated from the two-time, spatially averaged correlation function obtained between diastolic and systolic M-mode lines. The collective results for each subgroup are (r) I = 0.79 +/- 0.11 (sem) and (r) II = 0.62 +/- 0.13 (sem). The analysis presented, albeit in a limited population, is indicative of a trend in accordance with qualitative observations of Birnholz and Farrell (1985). M-mode analysis, as indicated by Adler et al. (1989) is a potentially useful technique to quantify such tissue motion.