Mulibrey nanism is an autosomal recessive disease with severe growth failure and multiple organ involvement. Heart manifestations include constrictive pericarditis and restrictive cardiomyopathy. The purpose of this study was to evaluate left ventricular (LV) diastolic and systolic function in children with mulibrey nanism utilizing two- and three- dimensional (2-D and 3-D) echocardiography and measurement of serum levels of natriuretic peptides. Of the 30 children diagnosed with mulibrey nanism in Finland, 26 participated. The control group comprised 26 children. In 2-D echocardiography, the interventricular septum and LV posterior wall were thicker in patients. The left atrium/aorta ratio measured a median 1.8 (range, 1.4–2.5) in patients and 1.3 (range, 1.0–1.7) in controls (p < 0.001). Patients differed from controls in several indices of diastolic LV function. In 3-D echocardiography, LV end diastolic volume measured a median of 51.9 ml/m² (range, 33.3–73.4) in patients and 59.7 ml/m² (range, 37.6–87.6) in controls (p = 0.040), and serum levels of N-terminal proatriopeptide and N-terminal pro-brain natriuretic peptide measured, respectively, a median of 0.54 nmol/L (range, 0.04–4.7) and 289 ng/L (range, 18–9170) in patients and 0.28 nmol/L (range, 0.09–0.72; p < 0.001) and 54 ng/L (range, 26–139; p < 0.001) in controls. They correlated with several indices of diastolic LV function. In a significant proportion of children with mulibrey nanism, myocardial function is impaired. Significant correlations appeared between indices of LV function, size of the left atrium, and levels of natriuretic peptides, showing that measurement of serum levels of natriuretic peptides is a useful follow-up method despite its dependence on loading conditions.