A method of making a cylindrical pressure vessel (11) with a large diameter port in its sidewall includes the step of providing a mandrel (23) of desired diameter and filament winding upon the same. After winding one overall innermost layer, an annular reinforcement belt (16) is helically wound atop a defined region using a band (60) of resin impregnated parallel strands (39) under tension. The annular belt (16) is then itself helically overwound with the resin impregnated parallel strands of filamentary material under tension to provide two complete outer layers. After curing and removal from the mandrel (23) at least one aperture (71) is cut in the sidewall within the reinforcement belt (16) and a side port fitting (75) is installed in the aperture (71).