In a review of 96 consecutive perinatal autopsies (42 nonautolyzed stillborn and 54 liveborn infants 12 h or less of age) with histologic sections of placental tissues and an undistended lung, we were impressed by the frequent occurrence of lymphocytic infiltrates in the interstitium of the lung. To study this phenomenon further we analyzed the cases for 56 clinicopathologic variables. Lymphocytic interstitial infiltrates were present in 22 of the cases, 5 stillborn and 17 liveborn. The severity of the infiltrates was highly significantly correlated with the severity of chorioamnionitis, funisitis, and fetal pneumonia (P < .001). A positive association was found with livebirth and a negative association with hyaline membrane disease (both P < .05). No significant association was found with gestational age, body size, or other complications of pregnancy, labor, delivery, or the immediate perinatal period. Twenty-seven cases had fetal pneumonia without lymphocytic interstitial infiltrates. Two cases had interstitial lymphocytic infiltrates in the absence of fetal pneumonia, but both had chorioamnionitis. Two other cases had numerous giant cells, shown to be of macrophage origin by immunoperoxidase stains, in the airways in addition to severe fetal pneumonia and lymphocytic interstitial pneumonia. The observations show that lymphocytic interstitial infiltrates and giant cells may be a feature of fetal pneumonia.