Objective To characterize the biological properties of HIV isolates obtained at the time of primary infection, and relate this to the subsequent course of the infection. Methods Syncytium-inducing (SI) capacity, tropism for cell lines (MT4 and H9) and replication rate were determined in 49 HIV isolates obtained from 17 HIV seroconverters. Thirteen of the 17 patients had a symptomatic primary HIV infection. Correlation between biological phenotype and clinical progression was analysed. Results SI isolates were recovered in six patients from the first sample taken during primary infection, non-SI (NSI) isolates only were identified in 10 of the patients and isolation culture was negative in one patient. For all patients from whom SI virus was initially isolated, this phenotype persisted during the follow-up period (range, 17–219 weeks). The duration of fever during primary infection was longer for patients with SI isolates than for patients with NSI isolates (P = 0.05). Both types of isolates were associated with a significant fall in CD4 lymphocytes during follow-up (P< 0.05). Patients with SI isolates developed HIV-related immune deficiency more rapidly than patients with NSI isolates (P < 0.05). Conclusions The biological phenotype of HIV strains isolated during the time of seroconversion may be an important determinant of the subsequent course of the infection.