SUMMARYThe purpose of this study was to DNA fingerprint the majority (64 %) of isolates received at the Pertussis Reference Laboratory during the 1993 whooping cough epidemic by pulsed field gel electrophoresis of Xba I -generated restriction digests. Two DNA restriction patterns, types 1 and 3, predominated (40% and 23 %, respectively, of 180 isolates) but type 2, identified in a previous study was notably absent. Twenty-one new DNA types occurred (24% of isolates), some being atypical as bands 155-230 kb were no longer conserved, but there was no statistically significant difference in their incidence in the upswing (JuneSeptember) compared to the downswing (October-December) phase of the epidemic. There was a relatively high proportion of new types, compared to type 1, at the peak (September). About 50 % of isolates received were from the North Western Region, where 44 % of isolates were DNA type 1. Whereas only 1 out of 10 isolates from Scotland were of this type, suggesting some geographic variation. Statistically significant findings included a higher proportion of isolates from female patients (P < 0-01), most marked in the 12-24 months age group (P < 0 05); a higher proportion of infants under 12 months requiring hospital admission compared to older children (P < 005); and a greater number of isolates from unvaccinated children (P < 0-01). Analysis of serotype according to four age groups (under 3 months, 3-12 months, 12-24 months and above 2 years) showed statistically significant differences (P < 0 05) with a noticeably lower proportion (38%) of serotype 1,3 in 3-12 months age group and higher prevalence (74%) of serotype 1,3 in the 12-24 months age group. There was no correlation between DNA type and serotype.