Solitary fibrous tumor (SFT) diagnosis in prostate can be challenging on small biopsies. Prostatic stromal tumors of unknown malignant potential (STUMP) and SFT have overlapping features. NAB2-STAT6 gene fusions that were recently identified in various SFTs lead to nuclear translocalization of STAT6. Nuclear STAT6 immunostaining is now considered an adjunct for SFT diagnosis. We evaluated STAT6 and an emerging sternness marker, ALDH1, in the differential diagnosis of SFT versus prostatic stromal lesions. Sixteen STUMPs, 12 SFTs, and 4 prostatic stromal sarcomas (12 needle biopsies, 13 radical prostatectomies, 7 transurethral resections) were retrieved (1995-2015). Sections were stained with polyclonal STAT6 antibody (Santa Cruz Biotechnology, Santa Cruz, CA; S20, 1:100) and monoclonal ALDH1 antibody (BD Biosciences, San Jose, CA; clone 44, 1:250). In STAT6 cases, only unequivocal nuclear staining (with/without cytoplasmic staining) was considered positive. Cytoplasmic ALDH1 staining was counted positive. Ten of 11 evaluable SFTs demonstrated strong and diffuse nuclear STAT6 positivity; 4 of 16 STUMPs had nuclear staining that was weak (1/4) or focal (1/4). ALDH1 positivity was seen in 10 of 12 evaluable SFTs and 3 of 15 STUMPs. Prostatic stromal sarcomas were STAT6 negative (4/4); 2 of 4 were ALDH1 positive. The sensitivity and specificity for STAT6 for the diagnosis of SFT were 91% and 75%, respectively. Coexpression of STAT6 and ALDH1 yielded the same sensitivity but improved the specificity (100%) for the diagnosis of SFT. STAT6 is a useful marker in the differential diagnosis of SFT versus STUMP. Using STAT6 and ALDH1 together increases specificity. STUMPs can show STAT6 positivity, and when they do, it is likely to be weak or focal. (C) 2016 Elsevier Inc. All rights reserved.