We retrospectively report four cases from two hospitals of nonpneumococcal pleural empyema with a likely false-positive result on the pneumococcal antigen test BinaxNOW (PATB) (Alere) performed in pleural fluid samples in patients with aspiration pneumonia risk factors. To determine whether the positive reaction was due to cross-reactivity, we separately tested the isolates from the pleural fluid samples, along with collection and reference strains. All patients had polymicrobial aerobic and anaerobic positive cultures, including Parvimonas micra in every case. In all cases, 16S rDNA polymerase chain reaction sequencing yielded Fusobacterium nucleatum. Samples for culture and specific polymerase chain reaction were negative for Streptococcus pneumoniae. We found that the false-positive PATB finding was likely due to P micra, a previously unknown cross-reactivity. In case of aspiration pneumonia risk factors, a positive PATB result must be interpreted with caution because there can be a false positivity due to anaerobic infection or co-infection.