[en] We present detonation copper cylinder test results on aged PBX 9404 (94 wt% HMX, 3 wt% CEF, 2.9 wt% NC, 0.1 wt% DPA) explosive. The charges were newly pressed from 37.5 year-old molding powder. We compare these results to equivalent data performed on the same lot when it was 3.5 years old. Comparison of the detonation energy inferred from detonation speed to that inferred from wall motion suggests that the HMX energy is unchanged but the NC energy has decreased to ∼25% of its original value. The degradation of explosives and their binders is a subject of continual interest. Secondary explosives such as HMX are sufficiently stable near room temperature that they do not measurably degrade over a period of at least several decades. For formulated systems the bigger concern is binder degradation, for which the three main issues are strength, initiation safety, and (if the binder is energetic) energy content. In this paper we examine the detonation energy of new and aged PBX 9404 (94 wt% HMX, 3 wt% tris-β chloroethylphosphate (CEF), 2.9 wt% nitrocellulose (NC), 0.1 wt% diphenylamine (DPA) (1, 2)), measured via the detonation copper cylinder test. In 1959, two independent PBX 9404 accidents (3) raised serious concerns about the safety of the formulation. Over about a decade's time, Los Alamos pursued a safer, energetically equivalent replacement, which ultimately became PBX 9501. In order to accurately compare the performance of the PBX 9404 and PBX 9501 formulations, W. Campbell and R. Engelke (C and E) developed a stringent cylinder test protocol that they called the Los Alamos Precision Cylinder Test (4). The present aging study is possible because excellent PBX 9404 data from those qualification tests endures.