Low‐grade inflammation, which plays important roles in the development of fatal diseases, is commonly observed in obese people. However, this has not been evaluated in lean people, who have relatively increased mortality risk compared with people of normal weight. Here, we elucidate the association between systemic low‐grade inflammation and low body weight, with particular emphasis on aging. We examined the relationship between circulating C‐reactive protein (CRP) and BMI in a cross‐sectional study of 2,675 apparently healthy adults who had undergone a medical check‐up. Overall, subjects with low BMI (<21.0 kg/m2, n = 585) showed a favorable cardiovascular profile without being undernourished. In the elderly (≥55 years old), logarithmic CRP (LogCRP) showed a sigmoid curve against BMI with a base at BMI 21.0–22.9 kg/m2, but not against waist circumference (WC), even in nonsmokers. In contrast, in middle‐aged people, LogCRP showed an almost linear relationship with both BMI and WC. LogCRP levels in elderly nonsmokers with low BMI, but not normal or high BMI, were significantly higher than those in middle‐aged with corresponding BMI (P < 0.05). After adjustment for age, sex, smoking status, and weight change over the past 2 years, the adjusted means of LogCRP still had a similar sigmoid curve against BMI in the elderly. These results suggest that elderly people with low body weight may have subtle low‐grade inflammation irrespective of a favorable cardiovascular risk, which remains to be confirmed in further studies.