Most studies on determinants of community-acquired pneumonia (CAP) in primary care have focused primarily on the elderly. Using a case-control study in four Dutch healthcare centres, determinants of CAP among children and young adults were identified. Cases included 156 young adults (aged 16-40 yrs) and 107 children (aged 0-15 yrs) diagnosed with CAP during 1999-2008. For each case, three controls were selected from the same age group. Separate logistic regression analyses were used to identify determinants in young adults and children. Lower age, asthma and previous upper respiratory tract infections (URTIs) were independently associated with CAP in children. Increasing age, asthma, three or more children at home, current smoking and three or more previous URTIs were independent determinants of CAP in young adults. The present study has three remarkable findings: 1) increasing age was an independent determinant of CAP in young adults; 2) having young children increased the risk of the development of CAP in young adults; and 3) the number of previous URTIs was independently associated with CAP in both children and young adults, possibly due to higher infection susceptibility. Further studies are required in order to better understand the aetiology of CAP and permit better diagnosis and treatment of this serious condition.