Single-family constructed wetland systems in Ohio, USA, are studied to evaluate their effectiveness in improving water quality. Twenty-one, three-cell systems (septic tank with two wetlands) are found to meet US Environmental Protection Agency (EPA) effluent load guidelines in 68% of the quarterly water quality samples collected from 1994 to 2001. These wetlands most frequently meet EPA standards for mitigation of biochemical oxygen demand (89% below 30 mg/l); total suspended solids (79% below 30 mg/l); and fecal coliform (74% below 1000 counts/100 ml). Phosphorus and ammonia discharge meet the guidelines less often (50% at 1 mg/l and 16% at 1.5 mg/l, respectively). These data also indicate that domestic treatment wetlands can reduce output of fecal coliform 88 9 27%, total suspended solids 56 953%, biochemical oxygen demand 70 948%, ammonia 56 931% and phosphorus 80 920%. Analysis of variance for these systems indicates that biochemical oxygen demand reduction is 10% less efficiently reduced during winter and ammonia was reduced 20% more efficiently in fall when compared with the other seasons. Phosphorus reductions display complex seasonal variations that imply that the least efficient phosphorus reduction occurs in winter and the most efficient reduction occurs in fall.