Purpose: The pharmacokinetic properties of mecillinam (MEC) for urinary tract infections are excellent, and the resistance rate in Enterobacteriaceae is low compared to other recommended antibiotics. The oral prodrug pivmecillinam (P-MEC) has been used successfully as first choice for cystitis in the Nordic countries for many years. Norwegian and Danish guidelines also recommend P-MEC for acute uncomplicated pyelonephritis (AUP) and intravenous (IV) MEC for suspected urosepsis (only in Denmark). Here, we wish to present an updated investigation on the clinical data behind these recommendations together with sparse but more current clinical data. Methods: Prospective clinical trials evaluating MEC as monotherapy or in polytherapy with one other beta-lactam (mostly ampicillin [AMP]) for pyelonephritis or bacteremia were reviewed. Outcomes of primary interest were clinical and bacteriological success and relapse, respectively. Search databases used were PubMed, Cochrane Library, and Embase. Results: Twelve clinical studies (1979-2015) were included in this integrated literature review. Clinical success was seen in 38/51 (75%) patients treated with MEC as monotherapy and in 152/164 (93%) patients treated with MEC and one other beta-lactam. Bacteriological success was seen in 35/47 (74%) and 117/167 (70%) patients treated with MEC alone and with one other beta-lactam, respectively. In complicated infections, bacteriological success was much lower. Clinical relapse rate was not well described. Several uropathogenic bacteremia cases were treated successfully with MEC alone (ie, 10/15 [67%] and 13/15 [87%] for clinical and bacteriological success, respectively) or with one other beta-lactam (ie, 57/65 [88%] and 53/63 [84%] for clinical and bacteriological success, respectively). However, data on bacteremia are very sparse. Adverse reactions were few and mild (73/406 [18%]) and primarily seen when AMP was co-administered (69/73 [95%]). No serious adverse reactions were reported. Conclusion: IV MEC or oral P-MEC for 14 days may be suitable for the treatment of AUP and pediatric pyelonephritis. Randomized controlled trials using a single standardized dose of P-MEC compared to other current recommendations are warranted. Similarly, more evidence is required before MEC should be recommended for bacteremia or sepsis due to Enterobacteriaceae.