Objective: According to the social zeitgeber theory, the lack of social support (SS) may decrease circadian rhythm regularity. However, the effect of SS on social rhythms in major depression has never been investigated. The objective of this study was to investigate the relation between SS and social rhythms in elderly patients with major depression. Methods: Case-control study on the relation of SS with social rhythm regularity in 213 elderly patients with major depressive disorder (MDD) and 183 elderly healthy comparison subjects (HCs). Social rhythm regularity was studied using the social rhythm metric (SRM-5), in which a lower score represents less regularity. SS was assessed with the social support list (SSL). Results: Patients with MDD displayed lower SRM-5 scores than HCs (4.94 +/- 0.94 versus 5.38 +/- 1.12; p = 0.003), as well as lower SSL-interactions (60.0 +/- 13.7 versus 70.5 +/- 11.6; p < 0.001), higher SSL-discrepancies (56.3 +/- 15.5 versus 39.4 +/- 7.2; p < 0.001), and higher SSL-negative interactions (11.0 +/- 4.5 versus 8.9 +/- 1.9; p < 0.001). In HCs, social support was negatively correlated with SRM-5 (SSL-interactions, r = -0.30; SSL-discrepancies, r = -0.23; SSL-negative interactions, r = -0.44). In MDD, SS was not correlated with SRM-5 (all r <= 0.03; all p > 0.05). Conclusions: Patients with MDD showed lower social rhythm regularity as well as lower measures of SS than HCs. In HCs, high SS was correlated with low social rhythm regularity, suggesting that increases in SS in combination with a healthy organization of circadian rhythms allow the social rhythms to become less rigid. Interestingly, in MDD, no correlation was found, suggesting that patients have a blunted response to social stimuli and may, therefore, benefit from treatment that increases the susceptibility to SS.