Dwarf mistletoe (Arceuthobium vaginatum subsp. cryptopodum (Engelm.) Hawksworth and Wiens) infestation results in severe loss of wood volume in the ponderosa pine forests (Pinus ponderosa Laws.) of Colorado. In this research equations were developed which describe the relationship between dwarf mistletoe incidence, severity and stand structure. The effect and inter-relationship between inter-tree competition and dwarf mistletoe was also described for irregular, multi-aged stands using growth and mortality of two-inch diameter classes as predetermined response variables. Stand structure had a significant effect (P $<$.05) on the relationship between dwarf mistletoe incidence and severity. The ratio of the percent trees infected to stand dwarf mistletoe rating (DMR) was significantly higher for even-aged stands than for multi-aged stands. A component of total stand basal area was used to describe inter-tree competition. BASL, which is the sum of the basal area of the trees in a given size class plus basal area of all trees of larger d.b.h., was a useful predictor of competition for small trees (d.b.h. $\leq$ 10 in.) but not for larger trees (d.b.h. $>$ 10 in.). For larger trees, stand basal area (BA) was a statistically significant (P $<$.05), but weak, predictor of competition. For small trees, BA was significant but not as good a predictor as BASL. Growth of trees of all sizes decreased steadily (beginning when trees were moderately infected) to 50% (at the heaviest infection levels) of the expected growth for uninfected trees. Percent of trees in a d.b.h. class heavily infected with dwarf mistletoe was positively correlated with 10 year mortality rates. For infected trees with d.b.h. $<$ 10 in., competition and d.b.h. were also useful predictors of mortality. A higher level of intertree competition (BASL) resulted in higher mortality rates while mortality declined exponentially with increasing tree d.b.h. The results show that stand density and structure interact together to impact growth and mortality.