Importance:​ Multiple osteochondromas is a rare hereditary skeletal disorder, characterized by bony protrusions arising from growth plates on long bones during skeletal development. The disorder frequently leads to diminished stature, deformities and functional limitations. Understanding of the natural history of multiple osteochondromas and its evolution in children and adolescents is limited.

Objective:​ To provide valuable information on the natural history of multiple osteochondromas, to inform recommendations for treatment and prevent impairments caused by osteochondromas.

Design:​ This retrospective cohort study in children with multiple osteochondromas includes longitudinal data collected from first to last follow-up visit for patient demographics, and over 36 months for disease evolution.

Setting:​ Data were collected from the Registry of Multiple Osteochondromas, which includes data from circa 1200 patients with multiple osteochondromas treated from 2003 to 2017 at IRCCS Istituto Ortopedico Rizzoli in Bologna.

Participants:​ Patients ≤18 years with multiple osteochondromas, who provided written informed consent and had data for ≥1 12-month follow-up visit.

Main outcome(s) and measurement(s):​ Demographics, clinical features, incidence of surgeries, and disease evolution (progression or regression) were assessed. Results were summarized using descriptive statistics, annual rates of new clinical features and surgeries, and Kaplan-Meier estimates. Patient height was evaluated following Italian growth charts.

Results:​ 158 patients were included in these analyses. Throughout follow-up, 80.4% of patients developed new osteochondromas, 57.6% developed new deformities, 23.4% developed new functional limitation(s). New osteochondroma(s) were developed by 28.5% patients by Month 12, 39.9% at Month 24, 50% at Month 36. Most new osteochondromas were detected in the younger population;​ patients aged 0–4 years underwent a significantly higher number of lesions within 12, 24 and 36 months of follow-up. The overall incidence of patients with ≥1 new deformity within 12 months was 17.7%, with incidences decreasing with increasing age (p = .023). In addition, the analyses on height highlight that 13 years is a cut off age for slow growth of the stature (p < .0005). At last follow-up visit, 46.2% of patients had disease progression, while regression (spontaneous and surgical) occurred in 7.6% (p = .007).

Conclusions and relevance:​ This natural history study reports the main set of clinically relevant data for patients with multiple osteochondromas during skeletal development, providing insight for patient management and development of therapeutic interventions.