A transmission cluster of HIV-1 M:B was identified in 11 patients with a median age of 52 (range 26-65) in North-East Germany by C2V4 region sequencing of the env gene of HIV-1, who-except of one-were not aware of any risky behaviour. The 10 male and 1 female patients deteriorated immunologically, according to their information made available, within 4 years after a putative HIV acquisition. Nucleic acid sequence analysis showed a R5 virus in all patients and in 7 of 11 a crown motif of the V3 loop, GPGSALFTT, which is found rarely. Analysis of formation of this cluster showed that there is still a huge discrepancy between awareness and behaviour regarding HIV transmission in middle-aged patients, and that a local outbreak can be detected by nucleic acid analysis of the hypervariable env region.