The objective of our study was to study which is the most accurate specimen for histological diagnosis of prosthetic joint infections (pseudocapsule or interface membrane). This is a prospective study including hip revision arthroplasties performed from January 2007 to June 2009. Specimens from pseudocapsule and from interface membrane were obtained from each patient. The histology was considered positive for infection when ≥5 neutrophils per high-power field ( × 40) were found. Definitive diagnosis of infection was considered when ≥2 cultures were positive for the same microorganism. According to the definition of infection, patients were classified in two groups: (A) patients with aseptic loosening in whom cultures obtained during surgery were negative and (B) patients with prosthetic joint infection. A total of 69 revisions were included in the study; 57 were classified in group A and 12 in group B. In group B, the percentage of positive interface membrane histology was significantly higher than the percentage of positive pseudocapsule histology (83 vs 42%, P=0.04, Fisher's exact test). The results suggest that periprosthetic interface membrane is the best specimen for the histological diagnosis of prosthetic joint infection.