Background: This study was conducted to assess whether patient-reported outcomes (PROs) differ by the underlying diagnosis (rheumatoid arthritis (RA)/inflammatory arthritis, osteoarthritis (OA), avascular necrosis of bone (AVN), other) in patients undergoing primary total hip arthroplasty (THA).Methods: We used prospectively collected data to assess the association of diagnosis with index hip function and pain. Moderate-severe activity limitation and moderate-severe pain were assessed at two- and five-year follow-up after primary THA using multivariable-adjusted logistic regression analyses. Odds ratios (OR) and 95% confidence intervals (CI) were calculated.Results: There were 5,707 primary THAs at two-years and 3,289 at five-years, 51% were women and the mean age was 65 years. The underlying diagnosis was RA in 3%, OA in 87%, AVN in 7% and other in 3%. In multivariable-adjusted analyses, compared to RA, diagnoses of OA and AVN were significantly associated with lower odds of moderate-severe activities of daily living limitations with an OR (95% CI) of 0.5 (0.3 to 0.8) (P = 0.01) and 0.4 (0.2 to 0.8) (P = 0.01), respectively, at two-years, but not at five-years, 0.7 (0.4 to 1.4) (P = 0.36) and 0.9 (0.4 to 1.8) (P = 0.78), respectively. At two-years, neither OA nor AVN were significantly associated with higher odds of moderate-severe pain (1.6 (0.6 to 4.5) (P = 0.40) and 2.8 (0.9 to 8.5) (P =0 0.06)), respectively. At five-years, AVN was associated with higher odds of moderate-severe pain with OR 4.1 (1.2 to 14.1) (P = 0.02), but not OA, 2.1 (0.7 to 6.5) (P = 0.22).Conclusions: We found that patients with OA and AVN had better functional outcomes and those with AVN worse pain outcomes after primary THA, compared to patients with RA/inflammatory arthritis. Insights into mediators of these relationships are needed to better understand these associations. © 2013 Singh and Lewallen; licensee BioMed Central Ltd.