The most convincing serologic evidence suggestive of a recent group A streptococcal infection is a doubling rise between acute and convalescent sera. Increases occur about 1 week after infection and peak 2-4 weeks later. In the absence of complication or reinfection, the ASO will usually fall to preinfection levels within 6-12 months. Final diagnosis is determined by both clinical and laboratory findings. If only a single specimen is available, or in the absence of a doubling rise (or fall) the upper `limit of normal' is a useful guide for determining the significance of the ASO. The upper `limit of normal' value represents the highest ASO level obtained in 85% of a `normal' population with no apparent recent streptococcal infection. Thus an ASO above the upper limit has an 85% chance of being due to a recent streptococcal infection. It should be recognized that an ASO on a single specimen below the upper limit of normal cannot definitely rule out a streptococcal infection since a significant increase may occur even though the levels do not exceed the `normal' range.