The COVID-19 is still raging worldwide. COVID-19 has shown severe pathogenicity in the elderly and people with underlying diseases. Thus, it is necessary to elucidate the pathology in patients with underlying diseases and in elderly patients. The use of an appropriate animal model is needed to overcome COVID-19; however, healthy young animals are usually used as experimental animals. Here, we discuss SARS-CoV-2 infection and related pathological conditions in cynomolgus monkeys including those with underlying diseases causing severe pathogenicity such as metabolic disease and advanced age. Cynomolgus macaques, with various clinical conditions and ages, were infected with SARS-CoV-2. There were differences between the pathologies of young and elderly macaques with underlying diseases. Consistent with humans, both viral RNA and infectious virus particles in pharyngeal swabs was higher in the aged macaques than in the young macaques. These were for a long period of time in the aged macaques. Inflammation due to pneumonia lasted for a longer period in the aged macaques. Therefore, the COVID-19 cynomolgus monkey model would be useful for elucidating COVID-19 pathophysiology, as well as, developing therapeutic and prophylactic agents against this disease.