Context: The natural history of pituitary incidentalomas (PIs) and nonfunctioning pituitary adenomas (NFPAs) remains poorly understood. Objective: The objective of the study was to synthesize the literature on the prognostic factors involved in the progression of PIs and NFPAs in patients followed up conservatively. Data Sources: We searched MEDLINE, EMBASE, and Cochrane CENTRAL. We sought to identify further studies by reviewing the reference lists from selected studies and reviews and by querying experts. Study Selection: Eligible studies were longitudinal observational cohort studies that enrolled patients with PIs/NFPAs and followed them up without any treatment from the time of detection and reported on mortality, lesion progression, and development of pituitary hormonal deficiency, apoplexy, or visual field defects. Data Extraction: Reviewers working independently and in duplicate determined studies' eligibility and collected descriptive, methodological quality, and outcome data. Event rates per 100 personyears (PYs) and associated 95% confidence intervals (CIs) were estimated from each study and pooled using the random-effects model. Data Synthesis: The 11 included studies had noncomparative single-cohort design. Follow-up duration ranged from 3 to 15 yr. There was a greater tendency for tumor growth in macroadenomas\ (12.5 per 100 PYs; 95% CI 7.9, 17.2) and in solid lesions (5.7 per 100 PYs; 95% CI 2.3, 9.2) in comparison with microadenomas (3.3 per 100 PYs; 95% CI 2.1, 4.5) and cystic lesions (0.05 per 100 PYs; 95% CI 0.0, 0.2). The development of pituitary apoplexy and worsening of visual field defects were rare. The overall incidence of new endocrine dysfunction was 2.4 per 100 PYs; 95% CI 0.0, 6.4. The majority of these analyses were associated with significant heterogeneity. There was a trend that did not reach statistical significance for greater incidence of pituitary apoplexy and new endocrine dysfunction worsening in macroadenomas compared with microadenomas. The quality of the evidence (risk of bias) was very low due to heterogeneity, methodological limitations, and imprecision caused by the small number of events. Conclusions: Despite the relatively high prevalence of PIs/NFPAs, the evidence on the natural history of these entities is scarce and of low quality. PIs/NFPAs seem to have fairly rare complications that may be more common when lesions are large (=10 mm) and solid. Copyright © 2011 by The Endocrine Society.