Computer data on 4721 consecutive patients patch tested with an extended standard series were analysed for the frequency of allergic reactions to each substance. Particular attention was given to the negative first reading at day 2 (D2) which later became positive at day 4 (D4). A total of 4210 allergic reactions were recorded. 993 of which were negative on D2 (24%). The most frequent positive reactions were to nickel sulphate (18%), fragrance-mix (8%), colophony (6%), neomycin (6%), cobalt chloride (6%), thiuram-mix (4%) and potassium dichromate (3%). The frequency of negative D2, positive D4 readings was sizeable for all but one substance, primin, with a range of 14% for fragrance-mix to 64% for neomycin. All of 21 positive reactions to primin were recorded on D2. In our opinion, it is unjustified to perform D2-only patch test readings.