Forty-three residents of 12 contiguous Connecticut communities were identified who had the onset of erythema chronicum migrans, Lyme arthritis, or both during the summer and fall of 1977. Nine of them (21%) remembered having been bitten by a tick at the site of the initial skin lesion a median of 12 days (range 3-20) before onset; one patient brought in the tick for identification (Ixodes scapularis). Compared to 64 of their neighbors, significantly more patients had cats and farm animals, and had noted ticks on their pets and tick bites on themselves. The incidence of the illness during 1977 was 2.8 cases per 1000 residents in the three communities on the east side of the Connecticut River, compared to 0.1 cases per 1000 residents in the nine communities on the west side, a difference of almost 30-fold. Taken with the results of a concomitant acarological study on both sides of the river, these findings support the hypothesis that erythema chronicum migrans and Lyme arthritis are tick-transmitted, specifically by I. scapularis.