A subsurface origin of 14C is rarely considered by researchers interested in dating groundwater. Theoretically, nuclear reactions induced by emissions from U and Th nuclei can produce measurable concentrations of 14C in groundwater. Under very special conditions, calculations indicate that subsurface production might cause significant errors in dates of less that 1¿104 years, although normally such errors showld not be important until after 5¿104 years. However, some anomalous 14C concentrations reported recently for deep groundwater may possibly be caused in part by subsurface production of 14C.