Background Serum markers of inflammation and muscle damage have shown clinical utility in some areas of medicine, but their value in determining the invasiveness or in predicting the early functional outcomes after total hip arthroplasty (THA) has not been demonstrated.Questions/purposes (1) Do serum markers of inflammation/muscle damage predict pain or early functional outcomes after contemporary THA performed through a direct anterior or miniposterior approach? (2) Do early functional outcomes as measured by in-hospital outcomes and clinical milestones differ between a contemporary direct anterior and miniposterior approach for THA? Methods Between August 31, 2013, and September 1, 2014, all patients presenting as candidates for THA at our institution who had not already had preoperative blood draws (161) were recruited for this study. Forty-two patients failed these exclusion criteria, eight patients declined enrollment, and 11 were consented but did not complete the required preoperative blood tests. Recruitment stopped when 50 patients had been enrolled in both the direct anterior group and the miniposterior group (2n = 100) based on a priori power analysis. One high-volume surgeon performed all of the direct anterior approaches and three high-volume surgeons performed the miniposterior approaches. Groups did not differ with the numbers available in mean age (63 years; SD 10; range, 35-86 years), sex (52% female), or mean body mass index (mean 31 kg/m 2 ; SD 7 kg/m 2 ; range, 20-73 kg/m 2 ). Serum markers measured including hemoglobin, hematocrit, myoglobin, creatine kinase (CK), C-reactive protein, interleukin-6, and tumor necrosis factor-a were collected at the preoperative clinic visit and on postoperative days 1 and 2 and compared with operative details, in-hospital complications, therapy progress, pain scores, and One of the authors (MWP) or a member of his immediate family, has or may receive payments or benefits, during the study period, an amount of USD 100,001 to USD 1,000,000 from Stryker Orthopedics (Mahwah, NJ, USA). One of the authors (RTT) or a member of his immediate family, has or may receive payments or benefits, during the study period, an amount of USD 10,000 to USD 100,000 from DePuy Orthopedics (Warsaw, IN, USA). One of the authors (RJS) or a member of his immediate family, has or may receive payments or benefits, during the study period, an amount of USD 10,000 to USD 100,000 from ZimmerBiomet (Warsaw, IN, USA) and USD 10,000 to USD 100,000 from Link Orthopaedics (Rockaway, NJ, USA). One of the authors (MJT) or a member of his immediate family has or may receive payments or benefits, during the study period, less than USD 10,000 from DJO Orthopedics (Vista, CA, USA). One of the authors (AFK) or a member of his immediate family, has or may receive payments or benefits, during the study period, an amount of less than USD 10,000 from Innomed (Savannah, GA, USA) and has or may receive payments or benefits, during the study period, an amount of less than USD 10,000 from ZimmerBiomet...