The prevalence and clinical signs of phocid herpesvirus type-1 (PhHV-1) infections among harbour seals (Phoca vitulina) in a seal rehabilitation centre in the Netherlands were monitored between June and September 1993 and 1994 when 34 and 36 seals, respectively, were rehabilitated. In both years PhHV-1-related disease outbreaks occurred in the pupping season. PhHV-1 infections were diagnosed by the demonstration of a more than four-fold increase in virus neutralising serum antibodies in paired serum samples, by the isolation of the virus from swab samples in primary seal kidney cells, and by the detection of PhHV-1 DNA with a polymerase chain reaction (PCR) assay in swab samples. This assay targets a 290 bp fragment of the glycoprotein D (gD) gene equivalent of PhHV-1. The PCR assay when combined with Southern blotting (PCR-SB) was approximately 1000 times more sensitive than virus isolation when tested with serially diluted samples from PhHV-1-infected cell cultures. In contrast with virus isolation, the PCR-SB scored as positive all the animals with serological evidence of PhHV-1 infection. The majority of seals present in the centre during the outbreaks contracted the infection and developed benign upper respiratory disease. However, the severity of PhHV-1-related disease was inversely correlated with age and fatal generalised infections occurred only in neonates.