The grading of soft tissue sarcomas is one of the most important prognostic factors and determines patient management. Although grading of most adult-type soft tissue sarcomas on biopsies correlates highly with the final grading on the excision specimen, it appears less reliable for tumors of smooth muscle. We assessed the pathologic findings for smooth muscle neoplasms diagnosed by core biopsy at our tertiary sarcoma center, and compared them with those in the subsequent excision specimens. A total of 100 patients with leiomyosarcoma first diagnosed on core biopsy and with a subsequent excision were identified and the accuracy of the biopsy grade determined by comparison with the excision grade. Differences in other salient histologic parameters were also noted. A grade difference between biopsy and excision specimens of leiomyosarcomas was found in 68% of cases, with all these cases showing an increase in grade from biopsy to excision specimen. Of the 3 parameters used for grading using the French Federation of Cancer Centers Sarcoma Group Grading System (FNCLCC), necrosis was the score that most commonly differed between biopsy and excision specimen (55%), closely followed by the mitotic count (51%). The grading of soft tissue smooth muscle tumor biopsies has a lower accuracy compared with other adult soft tissue sarcomas and should therefore be taken with caution, particularly as this may be an underrepresentation of the true tumor grade.