Despite the identification of acquired genetic mutations associated with Myeloproliferative Neoplasms (MPNs) there is a paucity of information relating to modifiable risk factors that may lead to these mutations. The MOSAICC Study was an exploratory case-control study of polycythemia vera (PV), essential thrombocythemia (ET), and Myelofibrosis (MF). MPN patients and population controls (identified by General Practitioners) and non-blood relative/friend controls were recruited from 2 large UK centers. Participants completed a telephone-based questionnaire analyzed by unconditional logistic regression analysis adjusting for potential confounders. Risk factors for MPNs identified included increasing childhood household density [odds ratio (OR) 2.55, 95% confidence interval (CI) 1.16-5.62], low childhood socioeconomic status (OR 2.30, 95%CI 1.02-5.18) and high pack years smoking (OR 2.19, 95%CI 1.03-4.66) and current smoking restricted to JAK2 positive PV cases (OR 3.73, 95%CI 1.06-13.15). Obesity was linked with ET (OR 2.59, 95%CI 1.02-6.58) confirming results in previous cohort studies. Receipt of multiple CT scans was associated with a strongly increased risk of MPN although with wide confidence intervals (OR 5.38, 95%CI 1.67-17.3). Alcohol intake was inversely associated with risk of PV (OR 0.41, 95%CI 0.19-0.92) and ET (OR 0.48, 95%CI 0.24-0.98). The associations with childhood household density, high pack years smoking and alcohol were also seen in multivariate analysis. This is the largest case control study in MPNs to date and confirms the previously reported associations with obesity and cigarette smoking from cohort studies in addition to novel associations. In particular, the role of smoking and JAK2 mutation cases merits further evaluation.