Objective: To report a case of heteropaternal superfecundation.
Design: Case report.
Setting: University paternity laboratory.
Patient(s): Father, mother, and a set of twins.
Intervention(s): Blood typing conventional markers, as well as polymerase chain reaction loci and restriction fragment length polymorphism loci of DNA.
Main outcome measure(s): Heteropaternal superfecundation was demonstrated after paternity investigation.
Result(s): The probability of paternity for twin 1 was 99.9999998%, whereas that for twin 2 was excluded on the basis of the following tests: Fy, Pi, human leukocyte antigen (HLA)-DQA1, D1S80, D17S5, HBGG, D5S110, D2S44, and D10S28.
Conclusion(s): Dizygous twins can have different biologic fathers, as demonstrated in this case. According to published data, the frequency of twins with different fathers is probably underestimated, at least in small selected populations such as those of paternity suits.