Osteomalacia has multiple aetiologies including the least common, tumour-induced osteomalacia (TIO). Recently, most cases of TIO have been confirmed to be due to phosphaturic mesenchymal tumour of mixed connective tissue type (PMTMCT). Most cases of TIO are the result of production of the fibroblast growth factor-23 (FGF-23) by the tumour. The authors recently showed reverse transcriptase PCR (RT-PCR) for FGF-23 to be valuable in the diagnosis of PMTMCT. However, the authors also noted FGF-23 expression in some cases of aneurysmal bone cyst (ABC) and chondromyxoid fibroma (CMF). For the present study, the authors studied FGF-23 expression by RT-PCR in 19 cases of ABC and eight cases of CMF, all with typical clinical and radiographic features and without evidence of TIO. Seven of 16 (44%) ABC and two of seven (29%) CMF were positive for FGF-23. These results confirm that ABC and CMF not uncommonly express FGF-23. These results strongly suggest caution and careful integration with all other clinical and radiographic data in the use of FGF-23 RT-PCR for the diagnosis of PMTMCT.