Dry wood which is re-wetted is at risk of attack by fungi. Mould and sapstain fungi on wood, while not significantly affecting strength properties, are considered unsightly and unacceptable in many markets, particularly those associated with high-value products. More recently, mould growth in buildings and the associated problems with odour, health and air quality have become issues. Additionally, there is concern that future international trade in wood products may be subject to restrictions regarding fungal pests present in wood. This study was set up to examine fungal growth on naturally infected kiln-dried dimension lumber. White spruce, lodgepole pine, alpine fir and western hemlock were exposed to a range of relative humidities (rh) between 75% and 95% and examined for fungal growth based on visual and microscope observation. Wood moisture content (mc) and fungal growth were determined weekly for ten weeks. The higher the rh, the earlier fungal growth appeared on the samples. Penicillium moulds predominated, with some dark fungi appearing with time on the wood at the highest rh. Once mould appeared, it progressed more rapidly at higher rh, affecting more of the surface and becoming more visible with time. Fungal growth was invisible to the unaided eye on all four wood species at 80% rh; it was seen on lodgepole pine after eight weeks at 82% rh but was not visible on alpine fir until six weeks at 86% rh, on spruce until seven weeks at 89% rh, or on hemlock until nine weeks at 90% rh. Lodgepole pine was 16% mc when mould became visible, while the mc for alpine fir was 19%, spruce was 21% mc and hemlock was 22% mc. Minute amounts of mould on lodgepole pine and spruce were visible only with a microscope after eight or nine weeks incubation at 72% or 74% rh and did not develop further. The first microscopic mould traces on hemlock and alpine fir occurred at 76% and 78% rh after six and ten weeks respectively. The study indicated that storage of kiln-dry lumber at 85-90% rh results in equilibrium moisture contents (emc) sufficient for mould growth and subsequent disfiguration of the product. At relative humidities between 75-85%, fungi may colonize and sporulate on the wood at non-visible levels, providing an infection source that can result in visible growth with slight increases in wood mc. If high-value kiln-dried products are subject to storage, transport or use under humid conditions, consideration should be given to application of a preventive treatment to control mould growth.