To verify a hypothesis that a relationship exists between the useful properties of the loose tube/optical fibers system and the structural features of poly(butylene terephthalate) (PBT) tubes, optical attenuation of optical fibers was studied both in the tubular optical fiber cable and in the tube proper. Examinations of the tube material structure was also performed by the DTA, SEM, and WAXD techniques. The results obtained allowed conclusion that during the standard industrial PBT tube extrusion process a finely dispersed crystalline a phase is formed in a quantity of similar to29%. The value is much less compared with the literature value of 66% (Yasuniwa et al., J Polym Sci B 1999, 37, 2420(1)) reported for a possible maximal content of the crystalline phase in PBT. Accordingly, it was assumed that the quantity of the crystalline phase in the tubes can be made to increase by their heating at a temperature above the glass transition temperature (T-g) of PBT. Results of the study corroborate the soundness of the assumption that during the optical cable conditioning process over its working temperature range, i.e., from -40 to 70degreesC, crystallisation of PBT tubes is likely to occur to result in their shrinkage which, as proven, affects the optical fibers attenuation, which is a major useful property of optical cables.