A simple microwave technique for in vivo monitoring of human pulmonary and cardiac activity is here presented. The technique is based on detecting the changes in the modulation envelope of amplitude modulated waves passing through the human body. A simplified human chest model was developed, proving an unambiguous correlation between heart blood filling and microwave transmission through the chest. A prototype system for transmittance measurement was realized at the 868.5-MHz operating frequency, demonstrating the feasibility of a small, low cost microwave plethysmograph. In vivo measurements showed a good agreement with numerical simulations.