The authors describe an outbreak of femoral hypoplasia or aplasia, isolated or associated with fibular or/and ulnar anomalies (Proximal femoral focal deficiency or FFU Complex). The epidemic seems to be limited in place (Rhone-Alpes region in France), and in time (starting late 1979 or early 1980 and apparently maintained during 1981 with an excess of winter-born patients). All the cases are sporadic and clinically strictly unilateral. An hereditary factor does not seem to play a role. No iatrogenic agent, no radiation exposure, no infectious diseases can be incriminated. The hypothesis of a responsible chemical teratogenic product suddenly introduced in the Rhone-Alpes region in 1979 is arising. The difficult inquiry is going on with finer analysis. This aggregation of unusual malformations is a typical example of what can be found at monitoring of birth defects.