BACKGROUND Fluorine‐18 fluoro‐deoxyglucose positron emission tomography (FDG‐PET) scanning has excellent sensitivity and specificity for staging non‐Hodgkin lymphomas, but to the authors' knowledge few studies to date have evaluated FDG‐PET in low‐grade lymphomas only. METHODS A retrospective study was performed on patients with biopsy‐proven nontransformed and transformed follicular lymphoma (FL), B‐cell small‐cell lymphocytic lymphoma (SLL/CLL), or marginal zone lymphoma (MZL) who underwent PET and computed tomography (CT) scans within 3 weeks. Standard uptake values (SUV) of all abnormal foci were measured. RESULTS In FL, PET demonstrated 94% sensitivity and 100% specificity for staging. PET was more specific than CT for detecting recurrence or assessing therapeutic responses (91% vs. 50%). FDG avidity among patients with WHO Grades 1, 2, and 3 disease was not significantly different (analysis of variance [ANOVA]). For MZL staging, PET had moderate sensitivity (71%) and outperformed CT alone in the depiction of extranodal sites (85% vs. 57% sensitivity). In SLL/CLL, PET sensitivity was 53% and underestimated disease extent in 5 of 19 patients (26%) compared with CT. PET did not affect initial management but confirmed suspected recurrences in 75% of patients. Nontransformed FL had a higher SUV (ANOVA, P<.05) compared with MZL and SLL/CLL. SUV was higher in transformed than in nontransformed tumors (P<.001, Student t test). CONCLUSIONS PET usefulness in staging low‐grade lymphomas varies depending on histology. PET sensitivity is excellent in FL and moderate in MZL. PET is more specific than CT for follow‐up in all types. PET has limited usefulness for SLL/CLL staging. However, a suggestive pattern of hazy and mild uptake was often noted in positive scans. In all low‐grade lymphomas, the emergence of foci of intense uptake should raise suspicion of conversion to high‐grade disease. Cancer 2006. © 2006 American Cancer Society.