Thymosin alpha 1 (TSN-a,) is produced by the thymus gland, regulates immune function, and measures thymic involution. Poor early growth permanently stunts size and function in features ceasing growth early, such as vertebral neural canals (VNCs) and the thymus gland (resulting in decreased TSN-a,). The purpose of this study was to follow-up prior research (Clark et al.:Hum. Biol. 60:435451, 1988) demonstrating that healthy adults (n = 16) with lower levels of TSN-a, had significantly smaller VNCs and greater sitting height. If true, these relationships should still exist later in time. Consequently, TSN-a, was remeasured in 13 of the original men 2.5 years later. Bivariate analyses showed that lower TSN-a, serum levels still tended to correlate with smaller VNCs (r = 52; P = .065). These results were similar to the original study (VNCs r = .47; P < .05). In a stepwise regression, age, weight, height, sitting height, fasting cortisol, and VNC were used to predict fasting TSN-a,. As in the first study, only sitting height and VNC entered the equation. Together, they explained 57% of the variation (P = .Ol), markedly similar to the 54% (P < .05) explained by the first study. Thus, even though this was a small sample, this stability over time strengthens the hypothesis that poor early growth permanently lowers TSN-a,. Nonetheless, these studies are pilot studies and should still be viewed with caution.