Background: MZL are uncommon B-cell lymphomas. The outcomes and prognostic factors of MZL remain poorly defined. Methods: We retrospectively identified 485 cases of MZL diagnosed between 1994-2014 from 2 institutions. Diagnoses had been confirmed by expert hematopathologists. We excluded 127 cases because of limited data and overlapping features with other lymphoma subtypes. Results: Of the 358 patients (pts) included in the analysis, 216 (60%) had extranodal MZL (EMZL) with most EMZL arising from stomach (32%), 56 (16%) nodal MZL (NMZL), 64 (18%) splenic MZL (SMZL), and 22 (6%) had typical MZL phenotype but were unclassifiable. Median age was 66 (range 13-95) years, 171 (48%) pts had stage III/IV disease at diagnosis. B-symptoms were present in 33 (9%) pts and 65 (18%) had > 4 nodal sites (LN) involved. Median time from diagnosis to treatment was 33 (2-1883) days and median duration of follow-up was 40.3 (2.2-236.4) months. Among treated pts, initial treatment was rituximab (R) (33%), chemotherapy +/- R (22%), radiation (16%), surgery (16%), and antibiotics (13%). Progression-free survival (PFS) and overall survival (OS) for the whole group was 5.15 (95% confidence intervals; C.I. = 2.8-10) and 19 (95% C.I. = 10-19) years respectively and there was no significant difference in PFS or OS between EMZL, NMZL, SMZL, or atypical MZL. On univariate analysis, age > 60 (p = 0.002), elevated serum lactate dehydrogenase (LDH) (p = 0.003), involvement of > 4 LN (p = 0.019), follicular lymphoma International Prognostic Index (FLIPI) score (p = 0.0005), and hemoglobin (Hb) level < 12 g/dL at presentation (p = 0.003) were associated with inferior OS. In a multivariate analysis, only age (hazard ratio (HR) = 5.48, p = 0.002) and Hb (HR = 2.97, p = 0.009) retained significance. Transformation to aggressive lymphoma occurred in 29 (8%) pts and was associated with high serum LDH and higher FLIPI at diagnosis. In pts treated with R +/- chemotherapy, R maintenance was associated with improved OS (p = 0.04). Conclusions: In our MZL series, one of the largest reported, prognosis of MZL is good with a median OS of over 19 years. Age > 60, Hb < 12 g/dL, elevated LDH, and > 4 LN involved are associated with inferior OS.