Experimental hyperthermia in pregnancy causes oromandibular and limb anomalies. In humans, hyperthermia has been suspected of being teratogenic. Two patients were investigated; both had limb defects and one had oromandibular anomalies. Their mothers had a febrile illness at about the 10th wk of gestation. The similarity of defects produced by experimental hyperthermia and those reported here is striking.