Weight has been shown to increase following total hip arthroplasty (THA). It is also reported that the preoperative weight does not correlate to the outcome of the surgery. Body mass index (BMI) is regarded as a better tool for analysis. However, only a few studies have analysed BMI, BMI change or its correlation to the outcome. We present a retrospective study of 78 consecutive patients undergoing elective primary cemented THA for osteoarthritis (OA) with a minimum 2-year follow-up from a single centre. Only 26% had an acceptable BMI at the time of surgery and at follow-up, there was a mean absolute weight increase of 2.5% and BMI increase of 2.1%. Fifty seven percent correctly predicted their weight change. BMI values were independent of the level of satisfaction or mobility using Western Ontario McMaster Universities OA Index (WOMAC) score and SF12 score. We conclude that the majority of patients undergoing primary THA will increase their BMI given sufficient follow-up irrespective of the outcome. Pre-operative BMI or BMI change post operatively is not a predictor of complications or mid term outcome of THA.