Little is known about the change in prevalence of comorbidities during the disease course of people with multiple sclerosis (MS) and whether the prevalences vary by MS onset type.To calculate the change in prevalence of comorbidities between symptom onset and the time of study, to compare the prevalences of comorbidities with those in the Australian population at the time of study and to examine onset-type differences.Comorbidity data from 1518 participants of the Australian MS Longitudinal Study and Australian population comparator data (2014-2015 National Health Survey) were used. The change in prevalence between time points and prevalence ratios (PR) at the time of study (crude, age and sex adjusted, and stratified by onset type) was calculated.Comorbidities were common, and those with the largest increases in prevalence between MS symptom onset and the time of study were depression (+ 26.9%), anxiety (+ 23.1%), hypertension (+ 21.9%), elevated cholesterol (+ 16.3%), osteoarthritis (+ 17.1%), eye diseases (+ 11.6%), osteoporosis (+ 10.9%) and cancer (+ 10.3%). Compared to the general population and after age and sex adjustment, participants had a significantly higher prevalence for 14/19 comorbidities at the time of study. The associations were strongest for anaemia, cancer (both PR > 4.00), anxiety, depression, migraine (all PR > 3.00), psoriasis and epilepsy (both PR > 2.00). No significant differences were seen by onset type.Comorbidities are common at MS symptom onset and increase with MS duration. Having MS may thus contribute to accrual of comorbidities. This emphasises the importance of optimal screening for and management of comorbidities in early MS and throughout the disease course.