Protein markers are commonly used in forensic medicine to establish the origin of human fluids detected in crime scenes. Semenogelins, the major protein constituents of semen coagulum, are the most effective markers for semen detection. Recently, it has been demonstrated that semenogelins are also ectopically expressed in small cell lung carcinomas (SCLC) and in a minority of non-small cell lung carcinomas (NSCLC). This finding prompted us to look for semenogelin expression in the serum of lung cancer patients. A set of 13 serum samples (3 from SCLC and 10 from NSCLC patients) was screened by enzyme-linked immunosorbent assay (ELISA), using a commercially available kit. Four of the NSCLC cases showed positive results. Ectopic expression of marker proteins in individuals affected by cancer could represent a potential source of forensic pitfalls.